(1.235)
- Introduce a helper function if_setflag() containing the code common
  to ifpromisc() and if_allmulti() instead of duplicating the code poorly,
  with different bugs.
- Call ifp->if_ioctl() in a consistent way: always use more compatible C
  syntax and check whether ifp->if_ioctl is not NULL prior to the call.

(1.237)
Rename equal() macro to sa_equal(), which matches the definitions
of sa_equal() in other files, and makes it more clear what equal()
is comparing.

(1.238)
In multicast routines:

Compare pointers with NULL rather than treating them as booleans.

Compare pointers with NULL rather than 0 to make it more clear
they are pointers.

Assign pointers value of NULL rather than 0 to make it more clear
they are pointers.