lazycatd.scm: Implement basic signal handling.

* lazycat-daemon/lazycat-daemon.c (react_to_terminal_signal)
  (set_lazycat_signals): New procedures.
  (inner_main): Export `c-set-lazycat-signals'.
* lazycat-daemon/lazycatd.scm (handle-signal): New procedure.
  (main): Setup signal handling.