previously, stdio used spinlocks, which would be unacceptable if we
ever add support for thread priorities, and which yielded
pathologically bad performance if an application attempted to use
flockfile on a key file as a major/primary locking mechanism.

i had held off on making this change for fear that it would hurt
performance in the non-threaded case, but actually support for
recursive locking had already inflicted that cost. by having the
internal locking functions store a flag indicating whether they need
to perform unlocking, rather than using the actual recursive lock
counter, i was able to combine the conditionals at unlock time,
eliminating any additional cost, and also avoid a nasty corner case
where a huge number of calls to ftrylockfile could cause deadlock
later at the point of internal locking.

this commit also fixes some issues with usage of pthread_self
conflicting with __attribute__((const)) which resulted in crashes with
some compiler versions/optimizations, mainly in flockfile prior to
pthread_create.

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.