Based on work from Florent Drouin, split the 32-bit link-layer type

Based on work from Florent Drouin, split the 32-bit link-layer type
field in a capture file into:

	a 16-bit link-layer type field (it's 16 bits in pcap-NG, and
	that'll probably be enough for the foreseeable future);

	a 10-bit "class" field, indicating the group of link-layer type
	values to which the link-layer type belongs - class 0 is for
	regular DLT_ values, and class 0x224 grandfathers in the NetBSD
	"raw address family" link-layer types;

	a 6-bit "extension" field, storing information about the
	capture, such an indication of whether the packets include an
	FCS and, if so, how many bytes of FCS are present.