the historic mktemp is supposed to blank the template string on
failure, rather than returning 0. just zero the first character so
that mkstemp and mkdtemp can still retry with O(1) space requirement.

note that this presently does not handle consistency of the libc's own
global state during forking. as per POSIX 2008, if the parent process
was threaded, the child process may only call async-signal-safe
functions until one of the exec-family functions is called, so the
current behavior is believed to be conformant even if non-ideal. it
may be improved at some later time.

use current time in nanoseconds and some potentially-random (if aslr
is enabled) pointer values for the initial tempfile name generation,
and step via a cheap linear prng on collisions. limit the number of
retry attempts to prevent denial of service attacks even if an
attacker can guess the filenames.

this is cleaner and makes it easy to impose a limit on the number of
retries later if it seems desirable to do so.

this allows sys/types.h to provide the pthread types, as required by
POSIX. this design also facilitates forcing ABI-compatible sizes in
the arch-specific alltypes.h, while eliminating the need for
developers changing the internals of the pthread types to poke around
with arch-specific headers they may not be able to test.

this is actually a workaround for a bug in gcc, whereby it asserts
inequality of the keys being compared...

It's not necessary to save any registers on the stack across syscall in
x86_64 __set_thread_area.  Don't waste cycles or bytes on it.

this was originally written for an early draft of the library where
non-standard functions would reside in a static library separate from
the shared libc.so, which would implement a pure standard. the idea
was not to depend on an implementation-dependent __syscall_ret
function in the main libc. but it turned out to be better to put
everything in a single library for both static and dynamic linking
uses, and thus the (incomplete) remnants of this feature were just
enlarging the source and binary.

- hide all the legacy xxxxxx32 name cruft in syscall.h so the actual
source files can be clean and uniform across all archs.

- cleanup llseek/lseek and mmap2/mmap handling for 32/64 bit systems

- alternate implementation for nice if the target lacks nice syscall