Strengthen the heuristics for pcap subtypes.

When trying to guess what type of capture a file is, look for as many
bogosities (caplen > len, microseconds >= 10^6/nanoseconds >= 10^9,
too-high caplen, too-high original len, caplen > snapshort length), to
increase the chances of guessing correctly.

(Every time somebody uses 0xa1b2c3d4 as the magic number for a capture
file that isn't standard pcap format, God kills a kitten.  Please, think
of the kittens.)

Change-Id: I3f397d598ed61dc82e2832be30452ebe8ace98e8
Reviewed-on: https://code.wireshark.org/review/3808
Reviewed-by: Guy Harris <guy@alum.mit.edu>