cxgbe(4): Do not run any sleepable code in the SIOCSIFFLAGS handler when
IFF_PROMISC or IFF_ALLMULTI is being flipped.  bpf(4) holds its global
mutex around ifpromisc in at least the bpf_dtor path.

MFC after:	3 days
Remove unnecessary inclusions of bsd.own.mk.
Include the kernel options after bsd.init.mk so that when we're
compiling on older systems that define all the MK variables in
bsd.own.mk we don't get the multiply defined error for options that
live in both.
Move most of the 15 variations on generating opt_inet.h and
opt_inet6.h into kmod.mk by forcing almost everybody to eat the same
dogfood. While at it, consolidate the opt_bpf.h and opt_mroute.h
targets here too.