Rename WTAP_ENCAP_ATM_SNIFFER to WTAP_ENCAP_ATM_PDUS, as it's not just

Rename WTAP_ENCAP_ATM_SNIFFER to WTAP_ENCAP_ATM_PDUS, as it's not just
used for the DOS-based ATM Sniffer.  (That's not a great name, but I
couldn't think of a better one.)

Add a new WTAP_ENCAP_ATM_PDUS_UNTRUNCATED encapsulation type for capture
files where reassembled frames don't have trailers, such as the AAL5
trailer, chopped off.  That's what at least some versions of the
Windows-based ATM Sniffer appear to have.

Map the ATM capture file type for NetXRay captures to
WTAP_ENCAP_ATM_PDUS_UNTRUNCATED, and put in stuff to fill in what we've
reverse-engineered, so far, for the pseudo-header; there's more that
needs to be done on it, e.g. getting the channel, AAL type, and traffic
type (or inferring them if they're not in the packet header).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@6840 f5534014-38df-0310-8fa8-9805f1628bb7