irc: add "server-time" in help of server option "capabilities"
core: fix modifier "weechat_print": discard only one line when several lines are displayed in same message (closes #171)

When several lines are displayed in a message (separated by "\n"), the
modifier "weechat_print" will now discard just one line (if return value is
an empty string), instead of discarding the whole message.
core: fix typo in French translation of /help set