Assume POSIX 1003.1-1988 or later for dirent.h.

Assume POSIX 1003.1-1988 or later for dirent.h.

* admin/CPP-DEFINES (HAVE_CLOSEDIR, HAVE_DIRENT_H): Remove.
* admin/notes/copyright: Adjust to src/ndir.h -> nt/inc/dirent.h renaming.
* configure.ac: Do not check for dirent.h or closdir.
* nt/inc/dirent.h: Rename from ../src/ndir.h, with these changes:
(struct dirent): Rename from struct direct.  All uses changed.
* nt/inc/sys/dir.h: Remove.
* src/dired.c: Assume HAVE_DIRENT_H.
(NAMLEN): Remove, replacing with ...
(dirent_namelen): New function.  All uses changed.  Use the GNU macro
_D_EXACT_NAMELEN if available, as it's faster than strlen.
(DIRENTRY): Remove, replacing all uses with 'struct dirent'.
(DIRENTRY_NONEMPTY): Remove.  All callers now assume it's nonzero.
* src/makefile.w32-in (DIR_H): Remove.  All uses replaced with
$(NT_INC)/dirent.h.
($(BLD)/w32.$(O)): Do not depend on $(SRC)/ndir.h.
* src/ndir.h: Rename to ../nt/inc/dirent.h.
* src/sysdep.h (closedir) [!HAVE_CLOSEDIR]: Remove.
Do not include <dirent.h>; no longer needed.
* src/w32.c: Include <dirent.h> rather than "ndir.h".

Fixes: debbugs:12958