this is the analog of commit dd8f02b7dce53d6b1c4282439f1636a2d63bee01,
but for putc.

for conformance, two functions should not have the same address. a
conforming program could use the addresses of getc and fgetc in ways
that assume they are distinct. normally i would just use a wrapper,
but these functions are so small and performance-critical that an
extra layer of function call could make the one that's a wrapper
nearly twice as slow, so I'm just duplicating the code instead.

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.