Keep track, in Wiretap, of whether the file is compressed, and provide

Keep track, in Wiretap, of whether the file is compressed, and provide
an API to fetch that.

When doing "Save" on a compressed file, write it out compressed.

In the Statistics -> Summary dialog and in capinfos, report whether the
file is gzip-compressed.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@42818 f5534014-38df-0310-8fa8-9805f1628bb7