Linux. The USB sniffing code for Linux now supplies a per-packet header
based on the one supplied by the Linux binary sniffing interface, so we
add a new DLT_ value and use that.
Fix his e-mail address, and add him to the credits.
Clean up some comments.
Protect all references to the USB stuff from the common Linux code with
PCAP_SUPPORT_USB, just as references to the Bluetooth code are protected
with PCAP_SUPPORT_BT.