Print both the name and the description of a link-layer type (the name

Print both the name and the description of a link-layer type (the name
isn't always very suggestive - for example, somebody might think
"EN10MB" is always 10 MB/s, and might not know that "IEEE802" is Token
Ring), using "pcap_datalink_val_to_description()".  Supply our own
"pcap_datalink_val_to_description()" if libpcap doesn't have it (even if
it has "pcap_datalink_name_to_val()").