sip_session2: when waiting for a greenlets to finish before exiting the program, print more informative message

darcs-hash:20090424090121-b2873-b48c75fbc2f053f42e10ebf2847625dfc5a59d85
sip_session2: add auto-answer option

darcs-hash:20090424090706-b2873-0f39e6ae663cfb5b29d8b8a17b6b26a519eede80