Give more details for --time-stamp-precision.
Allow builds if libpcap doesn't have pcap_set_tstamp_precision().

Check for pcap_set_tstamp_precision() in the configure script and, if
it's not there, don't include the code that allows time stamp precisions
to be set.
Don't assume the NFS request is aligned on a 4-byte boundary.

The XID is a multi-byte field, and, if it's not aligned, it must be
copied with UNALIGNED_MEMCPY().

This should fix GitHub issue #395.
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.

This (plus the corresponding change to libpcap) should fix GitHub issue
Boost the maximum snapshot length to 262144, for USBPcap.