Make wide char versions of opendir and friends.

	Make wide char versions of opendir and friends.
	* include/dirent.h (_wdirent, _WDIR): Define wide versions of
	struct dirent, DIR.
	(_wopendir,_wreaddir,_wclosedir,_wrewinddir,_wtelldir,
	_wseekdir): Add prototypes for wide versions of corresponding
	standard functions.
	* include/tchar.h; Add _UNICODE mappings for dirent.h
	structures and functions.
	* mingwex/dirent.c: Make _UNICODE neutral.
	* mingwex/wdirent.c: New file to define _UNICODE before
	including dirent.c.
	* mingwex/Makefile.in (DISTFILES): Add wdirent.c/
	(POSIX_OBJS): Add wdirent.o.
	(wdirent.o): Specify dependency on dirent.c as well as
	wdirent.c.
	* samples/dirent/wtest.c: New file, wide version of test.c.