care is taken that the setting of errno correctly reflects underflow
condition. scanning exact denormal values does not result in ERANGE,
nor does scanning values (such as the usual string definition of
FLT_MIN) which are actually less than the smallest normal number but
which round to a normal result.

only the decimal case is handled so far; hex float require a separate
fix to come later.

in principle this should just be an optimization, but it happens to
also fix a nasty bug where values like 0.00000000001 were getting
caught by the early zero detection path and wrongly scanned as zero.

bug detected by glib test suite

this caused glib to try to allocate >2gb for getpwnam_r, and probably
numerous other problems.

- add the rest of the junk traditionally in sys/param.h
- add prototypes for some nonstandard functions
- add _GNU_SOURCE to their source files so the compiler can check proto

this was basically harmless, but could have resulted in misreading
inputs with more than a few gigabytes worth of digits..

this also includes a related fix for vswscanf's read function, which
was returning a spurious (uninitialized) character for empty strings.

this code worked in strtod, but not in scanf. more evidence that i
should design a better interface for discarding multiple tail
characters than just calling unget repeatedly...

at this point, strto* and all scanf family functions are using the new
unified integer and floating point parser/converter code.

the wide scanf is largely a wrapper for ordinary byte-based scanf;
since numbers can only contain ascii characters, only strings need to
be handled specially.

assuming other code is correct, this should be a no-op, but better to
be safe...

vfprintf temporarily swaps in a local buffer (for the duration of the
operation) when the target stream is unbuffered; this both simplifies
the implementation of functions like dprintf (they don't need their
own buffers) and eliminates the pathologically bad performance of
writing the formatted output with one or more write syscalls per
formatting field.

in cases like dprintf where we are dealing with a virgin FILE
structure, everything worked correctly. however for long-lived files
(like stderr), it's possible that the buffer bounds were already set
for the internal zero-size buffer. on the next write, __stdio_write
would pick up and use the new buffer provided by vfprintf, but the
bound (wend) field was still pointing at the internal zero-size
buffer's end. this in turn allowed unbounded writes to the temporary
buffer.

the l prefix is redundant/no-op with printf, since default promotions
always promote floats to double; however, it is valid, and printf was
wrongly rejecting it.

not heavily tested but these functions appear to work correctly

shunget cannot unget eof status, causing wcstol to leave endptr
pointing to the wrong place when scanning, for example, L"0x". cheap
fix is to make the read function provide an infinite stream of bogus
characters rather than eof. really this is something of a design flaw
in how the shgetc system is used for strto* and wcsto*; in the long
term, I believe multi-character unget should be scrapped and replaced
with a function that can subtract from the f->shcnt counter.

advantages over the old code:
- correct results for floating point (old code was bogus)
- wide/regular scanf separated so scanf does not pull in wide code
- well-defined behavior on integers that overflow dest type
- support for %[a-b] ranges with %[ (impl-defined by widely used)
- no intermediate conversion of fmt string to wide string
- cleaner, easier to share code with strto* functions
- better standards conformance for corner cases

the old code remains in the source tree, as the wide versions of the
scanf-family functions are still using it. it will be removed when no
longer needed.

this is needed for upcoming new scanf

I'm not sure if it's legal for wordexp to modify this field, but this
is the only easy/straightforward fix, and applications should not
care. if it's an issue, i can work out a different (but more complex)
solution later.

this off-by-one error was causing values with just one digit past the
decimal point to be treated by the integer case. in many cases it
would yield the correct result, but if expressions are evaluated in
excess precision, double rounding may occur.