deal more gracefully with missing key files

[lsf37]

If for some reason the key file for a lock is missing, there is no way to get out of this situation just using mq.sh.

It should be safe to just rm the corresponding lock file, but we don't want users to have to do that manually.

The situation should be extremely rare, but it is not entirely impossible, so at least info and signal should deal with it gracefully.

[wom-bat]

mq.sh sem -signal -f foo
should work even if the key file is missing.

[lsf37]

I think that didn't work for me last time I tried it.