use proc.spawn_greenlet where powers of proc.spawn are unnecessary

darcs-hash:20090417104616-b2873-8c6ca4bf8546623f1e270c0e206e116fa51506ce
session2/sip_session2: support incoming connections

darcs-hash:20090417104715-b2873-4a592f695cd5df70d65ab0bed4efe26d79900622