Graceful handling of PCAPNG option comment UTF-8 decoding errors

[Schwaggot]

I believe before the current PCAPNG standard it was customary to have zero-terminated C-strings as comments, similar to the PCAP format. Although by now deprecated and switched to UTF-8 strings there are still capture files out there that have this old format. At the moment dpkt would just crash with a UnicodeDecodeError while trying to read these comments, which is a bit harsh for a field that has usually no contribution to the capture itself.

When merged this will add gracefull support for UTF-8 decoding errors, i.e., it will first try to parse comments as UTF-8 (new standard), if that fails it will check whether the data contains a zero-terminator and interpret it as a C-string. If no zero-terminator is found, the UnicodeDecodeError will still be raised (as previously).

[coveralls]

Coverage increased (+0.0004%) to 99.816% when pulling 03ec7fc on Schwaggot:graceful-comment-utf-8-decoding into 23dfff7 on kbandla:master.

[Schwaggot]

Added unit tests.