Don't treat 65535 as the maximum snapshot length.

Make it 131072, instead; the MTU on the Linux loopback interface, in at
least some versions of the kernel, is 65536, and that doesn't count the
fake Ethernet header, so we need a value bigger than 65536.  We don't
want a value that's *too* large, so that it causes attempts to allocate
huge amounts of memory, however.

Make it a #define while we're at it.
Boost the maximum snapshot length to 262144, for USBPcap.
Expand a comment (extra information taken from tcpdump's equivalent comment).