this was a POSIX requirement that was always in conflict with ISO C,
which specified a well-defined behavior for snprintf and swprintf so
long as the actual number of bytes/characters produced did not exceed
INT_MAX.
I originally raised this conflict for snprintf with the Austin Group
as tracker issue 761, which was never resolved. it was later reported
again as issue 1219, and as a result the conflicting requirement has
been removed.
the corresponding issue with swprintf does not seem to have been
addressed, but as the same reasoning applies to it, I am removing the
limitation on n for swprintf as well.

the old snprintf design setup the FILE buffer pointers to point
directly into the destination buffer; if n was actually larger than
the buffer size, the pointer arithmetic to compute the buffer end
pointer was undefined. this affected sprintf, which is implemented in
terms of snprintf, as well as some unusual but valid direct uses of
snprintf.
instead, setup the FILE as unbuffered and have its write function
memcpy to the destination. the printf core sets up its own temporary
buffer for unbuffered streams.

this header evolved to facilitate the extremely lazy practice of
omitting explicit includes of the necessary headers in individual
stdio source files; not only was this sloppy, but it also increased
build time.
now, stdio_impl.h is only including the headers it needs for its own
use; any further headers needed by source files are included directly
where needed.

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

the biggest change in this commit is that stdio now uses readv to fill
the caller's buffer and the FILE buffer with a single syscall, and
likewise writev to flush the FILE buffer and write out the caller's
buffer in a single syscall.
making this change required fundamental architectural changes to
stdio, so i also made a number of other improvements in the process:
- the implementation no longer assumes that further io will fail
 following errors, and no longer blocks io when the error flag is set
 (though the latter could easily be changed back if desired)
- unbuffered mode is no longer implemented as a one-byte buffer. as a
 consequence, scanf unreading has to use ungetc, to the unget buffer
 has been enlarged to hold at least 2 wide characters.
- the FILE structure has been rearranged to maintain the locations of
 the fields that might be used in glibc getc/putc type macros, while
 shrinking the structure to save some space.
- error cases for fflush, fseek, etc. should be more correct.
- library-internal macros are used for getc_unlocked and putc_unlocked
 now, eliminating some ugly code duplication. __uflow and __overflow
 are no longer used anywhere but these macros. switch to read or
 write mode is also separated so the code can be better shared, e.g.
 with ungetc.
- lots of other small things.

sadly the C language does not specify any such implicit conversion, so
this is not a matter of just fixing warnings (as gcc treats it) but
actual errors. i would like to revisit a number of these changes and
possibly revise the types used to reduce the number of casts required.