The SITA code in pcap-linux.c shares very little with the Linux code;

The SITA code in pcap-linux.c shares very little with the Linux code;
move it into pcap-sita.c, and make --with-sita set the pcap type to
"sita", so we build pcap-sita.c instead of, rather than in addition to,
pcap-linux.c.

Use "bpf_u_int32" rather than "ulong" in the SITA code, as it's intended
to be 32 bits long (the "l" in "htonl()" and "ntohl()" is historical -
they work on 32-bit quantities, and the "l" dates back to the days when
32-bit processors were a bit newer and 16-bit Unix was more common).

Those changes also, at least in theory, makes the SITA support work on
other Unix-compatible platforms; note that in README.sita.

Clean up pcap-sita.c, making routines no longer called outside it
static, folding trivial wrappers, and fixing various warnings.

Put the routines used by fad-sita.c and defined by pcap-sita.c into
pcap-sita.h.  Remove from pcap-sita.h the files that are now static to
pcap-sita.c.  Include pcap-sita.h in both fad-sita.c and pcap-sita.c, so
that we do cross-file prototype checking.