sip_session: improve error messages

darcs-hash:20090407122052-b2873-73d3d26196fa932b20eae1f0860defb7501c2258
sip_session: add :trace command, e.g. ":trace sip" or ":trace pjsip"

- allow user to type only the prefix, so that ":trace n" is possible instead of ":trace notifications"
- this also works in other places, i.e. ":call 3333 a"

darcs-hash:20090407122127-b2873-fd2708b311e45268a78915fbcb76759bfb9ed03c