<net/if.h>: Do not include <net/if_var.h> for _KERNEL

<net/if.h>: Do not include <net/if_var.h> for _KERNEL

* Clean up an ancient leftover: do not include <net/if_var.h> from <net/if.h>
  for kernel stuffs.

* Adjust various files to include the necessary <net/if_var.h> header.

NOTE:
I have also tested removing the inclusion of <net/if.h> from <net/if_var.h>,
therefore add <net/if.h> inclusion for those files that need it but only
included <net/if_var.h>.  For some files, the header inclusion orderings are
also adjusted.