sip_session: use Engine.start_cfg()

darcs-hash:20090325115129-b2873-1fff977820cb7e4d6c704d7fa7941cc5bfdad8fc
remove sipsimple.enginecfg (replaced by Engine.start_cfg() method)

darcs-hash:20090325115151-b2873-0786739d45636e55c02b3e84d1f80d034b484fb3