core: allow disabling plugin unloading with an env variable

By setting LICQ_NO_DLCLOSE=1 in the environment licq will not unload the
plugins, which makes it eaiser to debug licq as symbols will be correctly
resolved.
jabber: use a common definition of TRACE with support for arguments
jabber: use convenience function in Message to get date for delayed delivery
jabber: update user's alias on server if it is changed