Fix panic on IPv4 address removal introduced in r265279.

Reported by:	Trond Endrestøl
MFC with:	r265279
Really sort out the headers.

sys/systm.h must always come after sys/param.h.

Remove sys/types.h which should never be included together with sys/param.h.

Add sys/malloc.h for correctness even if it seems to don't be needed.

Remove more unused headers found by unusedinc (from bde@) and tested with a
universe build.

Reported by:	bde