Add new features to logger plugin (command /logger, log level, level by buffer, mask by buffer, ..), fix some bugs

New features:
- new command /logger
- log level, to log only some messages, according to importance (task #8592)
- level by buffer: custom level for some buffers (or group of buffers)
- log filename mask by buffer (or group of buffers)
- marker line is added after display of backlog
- add "delete" callback for config file sections
- add "mkdir_parents" function to plugin API
- remove old log options in IRC plugin

Bug fix:
- marker line is set only when user switches buffer (not when a plugin force
  switch, like IRC plugin does when opening server or channel buffer)
- backlog fixed (sometimes lines were not properly displayed)

src/plugins/irc/irc-protocol.h

@@ -70,6 +70,7 @@ struct t_irc_protocol_msg
 };
 
 extern char *irc_protocol_get_nick_from_host (const char *host);
+extern char *irc_protocol_tags (const char *command, const char *tags);
 extern void irc_protocol_recv_command (struct t_irc_server *server,
                                        const char *entire_line,
                                        const char *host, const char *command,