Merge if.c:1.235 from HEAD to RELENG_6:

• 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.

Discussed with: yar
Approved by: re (scottl)