Improve "rmdir --ignore-fail-on-non-empty"

Improve "rmdir --ignore-fail-on-non-empty"

* src/rmdir.c (remove_parents, main): With --ignore-fail-on-non-empty,
suppress a diagnostic also for other errno values, which can arise
with read-only media or when the parent directory has the immutable
attribute (set via chattr +i).
(errno_may_be_empty, ignorable_failure): New functions.
* src/remove.c (is_empty_dir): Move function to ...
* src/system.h (is_empty_dir): ...here, and make it inline.
Suggested by Josselin Mouette in <http://bugs.debian.org/363011>
via Bob Proulx.
* NEWS: Mention the improvement.