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

* lazycat-daemon/proxy-list.scm: Turn to singleton.
  (set-default-proxy-list!): New procedure.
  (proxy-list-load): Change syntax.  All callers updated.
  (proxy-list-get-proxy): Likewise.
  (proxy-list-get-list): Likewise.
  (proxy-list-stop-all): Likewise.
* lazycat-daemon/curiosity.scm (curiosity): Update.
* lazycat-daemon/lazycatd.scm (periodical-ping, initialize)
  (handle-stop-req, exec-cmd-on-host, periodical-ping, main):
  Update.