PDCLIB-2 PDCLIB-12: If the internal character conversion routines are called

with p_outbuf == NULL, then they will perform the conversion but *not* store
the result. This facilitates the implementation of mbrlen and similar
functions.

Added (optional) UTF-16 variants of the conversion routines. If provided them,
then PDCLib will use them whenever it needs a UTF-16 conversion (e.g. uchar.h
UTF-16 routines, or wchar_t on win32)