FBSD: catch up to 1997 and include if_var.h with if.h

The commit message for upstream's r257244 change includes:
 - Make the prophecy from 1997 happen and remove if_var.h inclusion
   from if.h.
Despite the clear public posting, we were caught unawares.  We made
it down to the cellar despite the missing stairs, but "Beware of
the Leopard" caused us to turn back, apparently.

Since if.h is included in many places and if_var.h is not present
on all OSes, pull the if.h inclusion into the common kernel headers
for afs/ and rx/ , and add in if_var.h (as well as the sys/socket.h
prerequisite).