Improvements to mqexec and key-based authentication

- mqexec now exits immediately on SIGHUP/SIGTERM if there are no
  running jobs. Otherwise, it logs how many jobs it's waiting for.
- mqexec filters now support a "not" clause, to match the inverse
  of a normal match
- mqexec now logs an error and exits if it cannot connect/bind to
  the given address for the jobs/results sockets
- The hash table that stores authorized keys in the NEB was
  refactored