libmount: add support for "bind,ro"

libmount: add support for "bind,ro"

Now it's necessary t use two mount(8) calls to create a read-only
mount:

  mount /foo /bar -o bind
  mount /bar -o remount,ro,bind

This patch allows to specify "bind,ro" and the remount is done
automatically by libmount by additional mount(2) syscall. It's not
atomic of course.

Signed-off-by: Karel Zak <kzak@redhat.com>