Add POSIX binary tree search API.

	Add POSIX binary tree search API.

	* mingwex/tfind.c: New file.
	* mingwex/tdelete.c: New file.
	* mingwex/tsearch.c: New file.
	* mingwex/twalk.c: New file.
	* mingwex/Makefile.in (DISTFILES): Add tsearch.c twalk.c tdelete.c tfind.c.
	* mingwex/Makefile.in (POSIX_OBJS): Add tsearch.o twalk.o tdelete.o tfind.o.
	* include/search.h (tfind): Declare.
	(tdelete): Declare.
	(tsearch): Declare.
	(twalk): Declare.
	(ENTRY): Define.
	(ACTION): Define.
	(VISIT): Define.
	(node_t): Define, on condition of _SEARCH_PRIVATE.