ui/lc.scm (handle-diff): Fix handling of `get-pattern' action.

* ui/lc.scm (handle-diff): Fix a bug: fix handling of command line
  arguments during `get-pattern' action.  Change invocation syntax.
  Print error if a command is not specified for `get-pattern'
  action.
periodical-ping.scm (ping-host): New procedure.

* lazycat-daemon/periodical-ping.scm (ping-host): New procedure.
  (periodical-ping): Use `ping-host'.