supervisor-main: Fix nodev oddity in Linux 3.17

Linux 3.17 (current extremely early git) implicitly sets and locks
nodev on new tmpfs mounts in a non-root userns.  This means that
remounting without nodev will fail now that the lock bits work
correctly.
Merge pull request #75 from amluto/master

supervisor-main: Fix nodev oddity in Linux 3.17