mkdir: fix bug when -m's more generous than umask

mkdir: fix bug when -m's more generous than umask

Problem reported by David McCall (Bug#45886).
I introduced this problem when fixing Bug#14371.
* NEWS: Mention the fix.
* src/mkdir.c (struct mkdir_options): New members umask_ancestor,
umask_self, replacing umask_value.
(make_ancestor): Use them when temporarily adjusting umask.
(main): Set them, and set the umask to umask_self instead
of leaving it alone.
* tests/mkdir/perm.sh (tests): Add test case for bug.