relay: remove temporary option "nicklistdiff" (in command init), send nicklist diff by default (weechat protocol)

The temporary option was introduced because the nicklist diff was experimental.
It is now enabled by default, and there is no way to disable it. Therefore any
relay client using weechat protocol must implement the message "_nicklist_diff"
(unless nicklist is ignored by client).

Extra note: a full nicklist may still be sent if a message with diff would be
bigger than whole nicklist. So both full nicklist (message "_nicklist") and
nicklist diff (message "_nicklist_diff") must be implemented by clients.
doc: add relay message "_nicklist_diff" in NEWS