bfd: Use MAP_FAILED for mmap failure

Use MAP_FAILED, instead of ((void *) -1), for mmap failure and use
((void *) -1) only if MAP_FAILED is undefined.

	* bfdio.c (bfd_mmap): Replace (void *) -1 with MAP_FAILED for
	mmap failure.
	* bfdwin.c: Don't include <sys/mman.h>.
	(MAP_FILE): Removed.
	(bfd_get_file_window): Replace (void *) -1 with MAP_FAILED for
	mmap failure.
	* cache.c: Don't include <sys/mman.h>.
	(cache_bmmap): Replace (void *) -1 with MAP_FAILED for mmap
	failure.
	* opncls.c (opncls_bmmap): Likewise.
	* sysdep.h: Include <sys/mman.h> if HAVE_MMAP is define.
	(MAP_FILE): New.  Defined as 0 if undefined.
	(MAP_FAILED): New.  Defined as ((void *) -1) if undefined.