session2: use 603 by default

darcs-hash:20090421091630-b2873-dd3cc0f85d15d128a4ef11edeb363ed77038d79d
sip_session2: skip identical registration messages

darcs-hash:20090421091709-b2873-fc9559564cf3a08dea2f06db203764d0ec7d1f5f