* fts.c: Include fts_.h first, to check interface.

* fts.c: Include fts_.h first, to check interface.
Do not include intprops.h; no longer needed.
Include cycle-check.h and hash.h, since fts_.h no longer does.
Remove unnecessary casts of closedir to void.
(fts_build): Use a simpler method (not involving TYPE_SIGNED) to
decide whether to decrement nlinks.
* fts_.h: Do not include hash.h or cycle-check.h; no longer needed.
(FTS): Use struct hash_table * instead of Hash_table, so that
we no longer need to include hash.h here.
* fts.m4 (gl_FUNC_FTS): Don't require AC_HEADER_STDC, as we
don't use its results.
Don't check for fcntl.h, stddef.h, stdlib.h, string.h, unistd.h,
since we include them unconditionally.  Don't require
AM_STDBOOL_H, since stdbool is a prerequisite.
Don't require AC_C_CONST, AC_TYPE_SIZE_T or check for ptrdiff_t
since we assume C89 or better.
Don't require AC_FUNC_CLOSEDIR_VOID, AC_FUNC_LSTAT, or AC_FUNC_STAT,
as we don't use their results.
Don't check for fchdir, memmove, memset, strrchr, as we use
them unconditionally.