This function is used by ping6 from iputils.

- remove the HAVE_EFFICIENT_IRINT case: fn is an exact integer, so
  it can be converted to int32_t a bit more efficiently than with a
  cast (the rounding mode change can be avoided), but musl does not
  support this case on any arch.
- __rem_pio2: use double_t where possible
- __rem_pio2f: use less assignments to avoid stores on i386
- use unsigned int bit manipulation (and union instead of macros)
- use hexfloat literals instead of named constants

some macros in sys/mtio.h and syslog.h used NULL without defining it

definition in linux:
 #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
where __O_TMPFILE and O_DIRECTORY are arch specific

and use _GNU_SOURCE || _BSD_SOURCE guards for all of the RFC 3678
namespace polluting things like glibc/uclibc does.

This way, if an fprintf fails, we get an incomplete group entry rather
than a corrupted one.

If *l == *r && *l, then by transitivity, *r.

previously:
timersub(&now, t, &diff);

warning: value computed is not used [-Wunused-value]

loop condition was incorrect and confusing and caused an infinite loop
when (broken) applications reaped the pid from a signal handler or
another thread before wordexp's call to waitpid could do so.

when WRDE_NOSPACE is returned, the we_wordv and we_wordc members must
be valid, because the interface contract allows them to return partial
results.

in the case of zero results (due either to resource exhaustion or a
zero-word input) the we_wordv array still should contain a terminating
null pointer and the initial we_offs null pointers. this is impossible
on resource exhaustion, so a correct application must presumably check
for a null pointer in we_wordv; POSIX however seems to ignore the
issue. the previous code may have crashed under this situation.

avoid using exit status to determine if a shell error occurred, since
broken programs may install SIGCHLD handlers which reap all zombies,
including ones that don't belong to them. using clone and __WCLONE
does not seem to work for avoiding this problem since exec resets the
exit signal to SIGCHLD.

instead, the new code uses a dummy word at the beginning of the
shell's output, which is ignored, to determine whether the command was
executed successfully. this also fixes a corner case where a word
string containing zero words was interpreted as a single zero-length
word rather than no words at all. POSIX does not seem to require this
case to be supported anyway, though.

in addition, the new code uses the correct retry idiom for waitpid to
ensure that spurious STOP/CONT signals in the child and/or EINTR in
the parent do not prevent successful wait for the child, and blocks
signals in the child.

* simplify sin_pi(x) (don't care about inexact here, the result is
  inexact anyway, and x is not so small to underflow)
* in lgammal add the previously removed special case for x==1 and
  x==2 (to fix the sign of zero in downward rounding mode)
* only define lgammal on supported long double platforms
* change tgamma so the generated code is a bit smaller

previously these macros wrongly had type double rather than long
double. I see no way an application could detect the error in C99, but
C11's _Generic can trivially detect it.

at the same time, even though these archs do not have excess
precision, the number of decimal places used to represent these
constants has been increased to 21 to be consistent with the decimal
representations used for the DBL_* macros.

this is enough to produce the correct value even if the constant is
interpreted as 80-bit extended precision, which matters on archs with
excess precision (FLT_EVAL_METHOD==2) under at least some
interpretations of the C standard. the shorter representations, while
correct if converted to the nominal precision at translation time,
could produce an incorrect value at extended precision, yielding
results such as (double)DBL_MAX != DBL_MAX.

this should not matter since the reality is that either all the sysv
sem syscalls are individual syscalls, or all of them are multiplexed
on the SYS_ipc syscall (depending on arch). but best to be consistent
anyway.

this is a Linux-specific extension to the sysv semaphore api.

siginfo_t is not available from signal.h when the strict ISO C feature
profile (e.g. passing -std=c99 to gcc without defining any other
feature test macros) is used, but the type is needed to declare
waitid. using sys/wait.h (or any POSIX headers) in strict ISO C mode
is an application bug, but in the interest of compatibility, it's best
to avoid producing gratuitous errors. the simplest fix I could find is
suppressing the declaration of waitid (and also signal.h inclusion,
since it's not needed for anything else) in this case, while still
exposing everything else in sys/wait.h

it's not clear why I originally wrote O_NOFOLLOW into this; I suspect
the reason was with an aim of making the function more general for
mapping partially or fully untrusted files provided by the user.
however, the timezone code already precludes use of absolute or
relative pathnames in suid/sgid programs, and disallows .. in
pathnames which are relative to one of the system timezone locations,
so there is no threat of opening a symlink which is not trusted by
appropriate user. since some users may wish to put symbolic links in
the zoneinfo directories to alias timezones, it seems preferable to
allow this.

the rest of the code is not prepared to handle an empty TZ string, so
falling back to __gmt ("GMT"), just as if TZ had been blank or unset,
is the preferable action.

try+l points to \0, so only one iteration was ever tried.

we need to skip to the second TZif header, which starts at
skip+44, and then skip another header (20 bytes) plus the following
6 32bit values.

if sizeof(time_t) == 8, this code path was missing the correct
offset into the zoneinfo file, using the header magic to do
offset calculations.
the 6 32bit fields to be read start at offset 20.

inet_aton returns a boolean success value, whereas __ipparse returns 0
on success and -1 on failure. also change the conditional in inet_addr
to be consistent with other uses of __ipparse where only negative
values are treated as failure.

now that we're waiting for the exit status of the child process, the
result can be conveyed in the exit status rather than via a pipe.
since the error value might not fit in 7 bits, a table is used to
translate possible meaningful error values to small integers.

I mistakenly assumed that clone without a signal produced processes
that would not become zombies; however, waitpid with __WCLONE is
required to release their pids.

while using "l" unconditionally gave the right behavior due to
matching sizes/representations, it was technically UB and produced
compiler warnings with format string checking.

i386 fenv code checks __hwcap for sse support, but in fesetround the sse
code was unconditionally jumped over after the test so the sse rounding
mode was never set.