Use an association list to store host attributes.

* lazycat-daemon/host-list.scm (host-list-load): Use an
  association list to store host attributes.
  (unroll-host, unroll-group): Likewise.
* ui/cli/pretty-format.scm (format-host-list): Likewise.
* ui/gtk/host-tree.scm (host-tree-add-host): Likewise.
* ui/gtk/lazy-gtk-cat.scm (handle-add-host): Likewise.
Implement periodical ping.

Implement periodical ping.  This functionality is quite unstable
yet, so it is disabled by default.  To enable the periodical ping
set "ping-interval" option to a value greater than zero.
* lazycat-daemon/host-list.scm (unroll-host): Rename to
  serialize-host.  All callers updated.
  (host-list-get-unrolled-list): Rename to
  host-list-get-serialized-list.  All callers updated.
  (unroll-group): Rename to serialize-group.  All callers updated.
  (serialize-group/state, serialize-host/state): New public methods.
* lazycat-daemon/host.scm: Add the status field.
* lazycat-daemon/lazycatd.scm: Add ping-interval option.
  (periodical-ping): New method.
* ui/cli/pretty-format.scm (format-host-list): Show hosts
  statuses.