umount: add --quiet option

umount: add --quiet option

The command umount supports things like --all-targets and --recursive
to umount all nodes in specified tree. Sometimes it makes sense to
aggressively use wildcards like /dev/sdb* and in this case --quiet
seems like a good choice

	umount --quiet --all-targets /dev/sdb*

to suppress 'not mounted' error messages. The new option suppress only
these error messages and nothing else.

Addresses: https://github.com/karelzak/util-linux/issues/672
Signed-off-by: Karel Zak <kzak@redhat.com>