rpki-validator.sh: add a run in foreground mode

Running "rpki-validator.sh run" will start the RPKI Validator in the
foreground (i.e. without it forking and detaching from the terminal).
This is the preferred behaviour when run from modern GNU/Linux init
systems such as systemd and upstart, which take care of daemonisation.
Merge pull request #4 from toreanderson/foreground-run-mode

rpki-validator.sh: add a run in foreground mode