these functions were written to handle clearing eof status, but failed
to account for the __toread function's handling of eof. with this
patch applied, __toread still returns EOF when the file is in eof
status, so that read operations will fail, but it also sets up valid
buffer pointers for read mode, which are set to the end of the buffer
rather than the beginning in order to make the whole buffer available
to ungetc/ungetwc.

minor changes to __uflow were needed since it's now possible to have
non-zero buffer pointers while in eof status. as made, these changes
remove a 'fast path' bypassing the function call to __toread, which
could be reintroduced with slightly different logic, but since
ordinary files have a syscall in f->read, optimizing the code path
does not seem worthwhile.

the __stdio_read function is also updated not to zero the read buffer
pointers on eof/error. while not necessary for correctness, this
change avoids the overhead of calling __toread in ungetc after
reaching eof, and it also reduces code size and increases consistency
with the fmemopen read operation which does not zero the pointers.

the purpose of this logic is to avoid linking __stdio_exit unless any
stdio reads (which might require repositioning the file offset at exit
time) or writes (which might require flushing at exit time) could have
been performed.

previously, exit called two wrapper functions for __stdio_exit named
__flush_on_exit and __seek_on_exit. both of these functions actually
performed both tasks (seek and flushing) by calling the underlying
__stdio_exit. in order to avoid doing this twice, an overridable data
object __towrite_used was used to cause __seek_on_exit to act as a nop
when __towrite was linked.

now, exit only makes one call, directly to __stdio_exit. this is
satisfiable by a weak dummy definition in exit.c, but the real
definition is pulled in by either __toread.c or __towrite.c through
their referencing a symbol which is defined only in __stdio_exit.c.

for seekable files, posix imposed requirements on the offset of the
underlying open file description after a stream is closed. this was
correctly handled (as a side effect of the unconditional fflush call)
when streams were explicitly closed by fclose, but was not handled
correctly at program exit time, where fflush(0) was being used.

the weak symbol hackery is to pull in __stdio_exit if either of
__toread or __towrite is used, but avoid calling it twice so we don't
have to keep extra state. the new __stdio_exit is a streamlined fflush
variant that avoids performing any unnecessary operations and which
never unlocks the files or open file list, so we can be sure no other
threads write new data to a stream's buffer after it's already
flushed.

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.