Add -L option to allow adjustment of original frame length. This change was motivate...

Add -L option to allow adjustment of original frame length.  This change was motivated by a question on ask where the user currently has to jump through hoops to accomplish the same thing which can now be done in 1 step via:

    editcap -T wpan -C 16 -L -F libpcap test.pcap test_wpan.pcap

I thought it would be useful enough for others as well.

Ref: http://ask.wireshark.org/questions/22689/problems-with-editcap-and-wpan-encapsulation-option

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@50491 f5534014-38df-0310-8fa8-9805f1628bb7