Alas, one can't count on CLNP packets with a destination NSEL of 0x21

Alas, one can't count on CLNP packets with a destination NSEL of 0x21
being COTP packets, so we make "dissect_cotp_internal()" return FALSE if
it saw no valid COTP packets, and return TRUE and set the COL_PROTOCOL
field to COTP if it saw at least one valid COTP packet, and have
"dissect_cotp()" dissect the packet as raw data if
"dissect_cotp_internal()" returns FALSE (for the benefit of callers such
as X.25).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@1861 f5534014-38df-0310-8fa8-9805f1628bb7