pcapng: save a per-interface snapshot length.

pcapng: save a per-interface snapshot length.

We don't currently use it, but 1) we would do so in the future if we
ever fully support pcapng and 2) Apple will do so if they pick up the
patch I'll provide once I file the bug originally reported as

    https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=16573

as an Apple bug, so this makes one less place where Apple's libpcap fork
would differ from our libpcap if they pick up 1.10 when it's released.

(This also keeps them from claiming that change is licensed under the
APSL, as we got it first, and none of *our* code is licensed under the
APSL.)