saslserv/main: Fix double free of pending sessions in _moddeinit()
saslserv: Register and unregister sasl mechs through the main module

Modules providing sasl mechs no longer come and go behind the back of saslserv/main
saslserv/main: Remove pending sessions on mech unregister
saslserv: Enable unload capability on all modules
saslserv/main: Log when the session list is non-empty on unload
Merge pull request #316 from attilamolnar/master+saslrework

saslserv: Rework registration of mechs, allow unloading of saslserv modules