Speak of the "-d encap:" value as a linktype, rather than a DLT; if a

Speak of the "-d encap:" value as a linktype, rather than a DLT; if a
name is specified, it currently must be a DLT_ name rather than a
LINKTYPE_ name, as we use libpcap to do the mapping and it currently has
no API to map LINKTYPE_ names to values, but if a number is specified,
it could either be a LINKTYPE_ name or a DLT_ name if the two are
different, and we want to encourage the use of LINKTYPE_ values.

Note that in comments.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@51204 f5534014-38df-0310-8fa8-9805f1628bb7