Avoid resource deadlocks when one domain has exhausted its memory. Attempt
other allowed domains if the requested domain is below the minimum paging
threshold. Block in fork only if all domains available to the forking
thread are below the severe threshold rather than any.

Submitted by: jeff
Reported by: mjg
Reviewed by: alc, kib, markj
Approved by: re (rgrimes)
Differential Revision: https://reviews.freebsd.org/D16191