svc - misc improvements

svc - misc improvements

* Disallow extremely dangerous command sequences like 'svc exit' without
  a label (which would wildcard everything).

* Allow the label to be specified as 'all' for when you really want to
  operate on everything with a dangerous command.

* Clean up the socket and pid files on normal exit/kill, even if the
  service is not present.

* Add 'help' directive and -h option to cover our bases.