fdopendir: work around FreeBSD bug

fdopendir: work around FreeBSD bug

Without a declaration, at least tar would core dump on 64-bit
FreeBSD because gcc only used 32 bits of the resulting pointer.

* m4/dirent_h.m4 (gl_DIRENT_H_DEFAULTS): New witness.
* m4/fdopendir.m4 (gl_FUNC_FDOPENDIR): Set it.
* modules/dirent (Makefile.am): Substitute it.
* lib/dirent.in.h (fdopendir): Supply missing FreeBSD
declaration.
* doc/posix-functions/fdopendir.texi (fdopendir): Document the
fix.
Reported by Christian Weisgerber <naddy@mips.inka.de>.

Signed-off-by: Eric Blake <eblake@redhat.com>