Some platforms that lack <netpacket/packet.h> define PF_PACKET but don't

Some platforms that lack <netpacket/packet.h> define PF_PACKET but don't
support PF_PACKET sockets, and, in particular, don't define in
<linux/if_packet.h> any of the stuff needed by the code to handle
PF_PACKET sockets.  Define HAVE_PF_PACKET_SOCKETS if either

	1) we have <netpacket/packet.h>

or

	2) PF_PACKET is defined *and* PACKET_HOST is defined by
	   <linux/if_packet.h>

and use HAVE_PF_PACKET_SOCKETS, not PF_PACKET, be what we use in #ifdefs
to conditionally compile in support for PF_PACKET sockets.

Not all platforms define ARPHRD_SIT, either; #define it if it's not
already defined.