sip_session: add :help command

- get rid of command -> function mapping, use a cmd_xxx convention

darcs-hash:20090330092041-b2873-d5ae54a3a53371f7b908ca8fc7fcf661f33f1427
Replaced remaining instances of RuntimeError with SessionStateError in session

darcs-hash:20090330093321-58103-054c608f681acf0ad92a0e8aa25e9d20be1ac096