Always allow '-o remount,ro'

Always allow '-o remount,ro'

Allow the mount(8) utility to always operate on all datasets when
remounting them read-only.  This critical for rc.sysinit/umountroot
which remounts the root filesystem read-only during shutdown to
ensure everything is correctly flushed to disk.

Fix minor typo, the check to set zfsutil should use the bitwise
'&'.  I must have accidentally hit the adjacent '*' and obviously
neither the compiler or my code review caught this.  Fix it now.