Isn't Linux fun? You might have a distribution whose kernel is a shiny

Isn't Linux fun?  You might have a distribution whose kernel is a shiny
new 2.2-or-later kernel, with PF_PACKET sockets including SOL_PACKET,
but with an older version of the C library whose headers don't define
SOL_PACKET.

On those systems, define SOL_PACKET to have the value it has in the
2.2.0 kernel; this means we can get rid of the "#ifdef
SOL_PACKET"/#endif stuff wrapped around the code to turn promiscuous
mode on or off (we don't want that #ifdeffed out, as if it's absent we
won't pay attention to the "promiscuous mode" flag argument to
"pcap_open_live()").