Handle DLT_NULL correctly - the AF_ value is in host byte order, which

Handle DLT_NULL correctly - the AF_ value is in host byte order, which
means that we should "htonl()" it before using it in BPF expressions
*but*, if we're reading a capture file from a machine with the opposite
byte order from ours, we should byte-swap it before "htonl()"ing it.

Handle OpenBSD DLT_LOOP as well - it's like DLT_NULL except that the AF_
value is in *network* byte order.

Don't support checking for inbound or outbound packets except on those
data link types that supply an inbound/outbound qualifier (DLT_SLIP and
DLT_PPP) - this came from OpenBSD's libpcap, delta 1.12 to "gencode.c".