privsep: Implement a resource limited sandbox

privsep: Implement a resource limited sandbox

For systems without Capsicum or Pledge we can create a resource
limited sandbox provided that either ppoll(2) or works with
RLIMIT_NOFILES set to zero.

As far as dhcpcd is concerned, that means Linux and Solaris
won't work with this, but NetBSD and DragonFlyBSD will.

To achieve this, a special control proxy process will be spawned
just to accept new connections over the control socket because
this *cannot* be limited by RLIMIT_NOFILES.