mkdir: don't assume umask equals POSIX default ACL mask

mkdir: don't assume umask equals POSIX default ACL mask

This fixes Bug#14371, reported by Killer Bassist.
* NEWS: Document this.
* src/mkdir.c (struct mkdir_options): Remove member ancestor_mode.
New member umask_value.  All uses changed.
* src/mkdir.c (make_ancestor): Fix umask assumption.
* src/mkdir.c, src/mkfifo.c, src/mknod.c (main):
Leave umask alone.  This requires invoking lchmod after creating
the file, which introduces a race condition, but this can't be
avoided on hosts with "POSIX" default ACLs, and there's no easy
way with network file systems to tell what kind of host the
directory is on.
* tests/local.mk (all_tests): Add tests/mkdir/p-acl.sh.
* tests/mkdir/p-acl.sh: New file.