lazycat-daemon/host-list.scm: Turn to singleton.

* lazycat-daemon/host-list.scm: Turn to singleton.
  (set-default-host-list!): New procedure.
  (host-list-empty?, host-list-load, serialize-host)
  (host-list-save, host-list-add-group, host-list-rem-group)
  (add-host, host-list-add-host, host-list-rem-host)
  (host-list-get-host-by-id, host-list-get-group-name-by-host-id)
  (get-group-by-host-id, host-list-get-plain-list)
  (host-list-get-serialized-list): Change syntax.  All callers
  updated.
  (serialize-host/state, serialize-group): Add comments.
* lazycat-daemon/curiosity.scm (curiosity): Update.
* lazycat-daemon/lazycatd.scm (initialize)
  (handle-add-req, handle-rem-req, handle-list-req)
  (exec-cmd-on-host, exec-cmd, handle-diff-req, main): Update.
* lazycat-daemon/periodical-ping.scm (periodical-ping): Update.