Improve the ping plugin

fix a TB in the MUC
fix the completion (no quotes)
add a timeout of 5 seconds
Do not ping twice in private tabs
Fix #2231 (update the plugins to use the new help system)

And fix some imprecisions/mistakes in the help.
Fix /reconnect
Fix #2132 (/disconnect)
Add a plugins_conf_dir option
Add help about multi-account