Set name for `curiosity' and `periodical-ping' threads.

* lazycat-daemon/lazycat-daemon.c (set_thread_name): New
  procedure.
  (init_prctl_module): New procedure.
  (inner_main): Define `prctl' module.
* lazycat-daemon/curiosity.scm (curiosity): Set thread name.
* lazycat-daemon/periodical-ping.scm (periodical-ping): Likewise.