Add support for writing pcap-ng files by specifying -n as an option.

Add support for writing pcap-ng files by specifying -n as an option.
Leave pcap the default file format for now. The is should be reconsidered
before the next release as it might make sense to use pcapgn as
the default as we do for dumpcap. (We can use the -P option to allow
switching back to pcap).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@46691 f5534014-38df-0310-8fa8-9805f1628bb7