Clean up printing of Q.933 packets.

Clean up printing of Q.933 packets.

Check the packet length and the snapshot length as we parse the packet.
Extract each field as we go.

Support arbitrary call reference lengths, 0 to 15 octets.

Handle single-octet IEs correctly - don't look for a length.

Handle both locking and non-locking shifts correctly.  Don't assume that
the first octet after the message type is a shift and contains a
codeset.  We were doing that, meaning that we tended to think codeset 1
was being used (by misparsing an IE with a code of 0x51 as a shift to
codeset 1) when it wasn't - codeset 0 was being used; correctly handle
codeset 0.