If open + fchdir fails, fall back on xgetcwd + chdir.

	If open + fchdir fails, fall back on xgetcwd + chdir.
	The old code tested for this at compile-time,
	but SunOS 4.1.4 fchdir can fail at run-time.

	* find/defs.h (fchdir): Define to -1 if not available.
	* find/defs.h (starting_dir, starting_desc):
	Always declare.  starting_dir now points to const.
	* find/find.c (starting_dir, starting_desc): Likewise.
	* find/find.c (starting_dir):
	Now "." if starting_desc is nonnegative, for benefit of diagnostics.
	(main, process_top_path, process_dir):
	If open + fchdir fails, fall back on xgetcwd + chdir.
	* find/pred.c (launch): Likewise.