Adds ejabberd_hosts:update_host_conf/2 to update a host with a new
configuration on the fly. Typically this is useful to enable / disable
specific modules for a given host without restarting the whole app.
Also fixes a problem with mod_disco, after config refreshing to include
global services if there are not locally available.
Thanks to:
Alexey Shchepin for planning and developing MH in late 2007
Geoff Cant for improving during 2008 and 2009
Christophe Romain and Badlop for merging into master in 2010