PDCLIB-15 PDCLIB-16:

 * Move _PDCLIB_file_t to _PDCLIB_io.h, compertmentalizing dependencies.
   Removes I/O specific structures from _PDCLIB_int.h. Allows _PDCLIB_io.h
   to depend upon _PDCLIB_threadconfig.h without recursive dependencies
   happening.
 * Add "lock" member to _PDCLIB_file_t as the stream locking mutex
 * Add flockfile/ftrylockfile/funlockfile as functions which manipulate the
   locking mutex explicitly
 * Add man page on flockfile/funlockfile/ftrylockfile
 * Add flockfile/ftrylockfile/funlockfile and *_unlocked to <stdio.h> and
   <wchar.h> as appropriate
 * win32: Initialize mutex member correctly