mon: fix command caps check

We must require something or else the caps check is going to pass in
a degenerate sense.  Use X for commands.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/MonCap: simplify rwx match logic

Make this a positive check instead of double negative.

Signed-off-by: Sage Weil <sage@inktank.com>
qa/workunits/mon/caps.sh: clean up users; rename

Signed-off-by: Sage Weil <sage@inktank.com>