domainset: Define additional global policies

Add global definitions for first-touch and interleave policies. The
former may be useful for UMA, which implements a similar policy without
using domainset iterators.

No functional change intended.

Reviewed by: mav
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29104

(cherry picked from commit 29bb6c19f044274cc6693f0c2463ca22106cd3ab)