rm-rf: introduce new flag REMOVE_MISSING_OK

rm-rf: introduce new flag REMOVE_MISSING_OK

This new flag suppresses error if the top-level path specified doesn't
exist. This is useful since suppressing this on the caller side isn't
easy, since ENOENT migh be propagate for some reason from further inside
and we can't distuingish that.

While we are at it, also be a bit more careful witht the various
combinations of flags.

(Note that in some cases rm_rf() was already ignoring ENOENT from
unlink() or rmdir(), however that was pretty useless, since we always
open() the top-level path with O_DIRECTORY and if that hit ENOENT we
didn't ignore the failure).