ostree-remount: Check for / being *mounted* read-only, not necessarily writable

The previous S_IMMUTABLE commit broke ostree-remount; / is now not
actually writable.  All we really wanted to know though was whether it
was *mounted* writable, so check that via statvfs() which is cleaner
anyways (i.e. not via access() which kernel people hate).

https://bugzilla.gnome.org/show_bug.cgi?id=728006