* Test cleanups: surround the code for all functions by #ifndef REGTEST

* Don't REGTEST internal functions (it doesn't make sense)
* <stdio.h>: add _PDCLIB_fdopen, which opens a file based upon its _PDCLIB_fd_t
  file descriptor
    TODO: Should this be public or not? POSIX fdopen demonstrates utility
    TODO: Can we stop storing the file name? Is that useful?
* fopen: rewrite based upon _PDCLIB_fdopen to consolidate code
* strndup: don't REGTEST - mingw doesn't implement.
* <stddef.h>: conditional definition of wchar_t for <wchar.h>
* <wchar.h>: Add 4 simple functions for dependencies
* <_PDCLIB_aux.h>: rename _PDCLIB_ALL to _PDCLIB_EXTENSIONS
* platform/example/functions/stdio/tmpfile.c: return NULL, not 1
* Build system: SubInclude the platform directory to enable the building of
  support routines (e.g. app startup files)
* Test system: If $(CRT0) is set, link the contents of it into the tests. This
  enables the platform code to provide startup files.