posix:spawn: Use getdents64 when closefrom is not available.

* configure.ac (AC_CHECK_FUNCS): Check getdents64.
* src/spawn-posix.c [!HAVE_CLOSEFROM] (get_max_fds): Use getdents64
instead of readdir, for AS-Safety.

closefrom is available with glibc >= 2.34.
getdents64 is available with glibc >= 2.30.
To support older glibc versions, we could use syscall as
GPGME does.

• GnuPG-bug-id: T7696
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>