allow bind mounts outside of chroots

If we're in a unique namespace, there's no reason to not allow bind
mounts.  These can be useful like when setting up a unique /dev, or
to make specific paths read-only for a daemon.

Bug: chromium:680859
Test: `sudo ./minijail0 -rvpd -b /dev/log,/dev/log /bin/logger asdf` works
Change-Id: I9dcb7f072dfdaee8b405e5d05e5d94ee999767e8
2 files changed