For Linux, add to the pcap_md structure a pointer to a memory-mapped

For Linux, add to the pcap_md structure a pointer to a memory-mapped
region and the size of the region; use that pointer rather than the bp
or buffer member (that means we don't have to worry about
pcap_cleanup_live_common() attempting to free that buffer).  Use the
saved size when unmapping the memory-mapped region.

Use that for Linux USB memory-mapped access as well - and unmap the
memory-mapped region when we close the pcap_t, because we *do* have to
unmap it.