Change a bunch of calls to col_append_*() to col_add_*() in places where we

Change a bunch of calls to col_append_*() to col_add_*() in places where we
_want_ what's currently in the INFO column (usually a more generic message
type from a lower level dissector) replaced (usually with a more
specific--higher level--message type from the currect dissector).  Add a
comment there not to change it back and to explain the use of col_set_fence()
in case someone finds data there that they _don't_ want replaced.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@22409 f5534014-38df-0310-8fa8-9805f1628bb7