Keep a "display name" as part of a capture_file structure; for live

Keep a "display name" as part of a capture_file structure; for live
capture temporary files, it's "<capture", and for saved capture files,
it's the last component of the pathname of the file.  Use that in
various places when displaying the file name.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@8474 f5534014-38df-0310-8fa8-9805f1628bb7