domainset(9): Split domainset validation logic into a separate function

This change splits the validation and 'struct domainset'-filling logic
from kern_cpuset_setdomain into a separate function - domainset_populate.
This function's main use is to validate user-provided domainset(9)
policies and populate a struct domainset before handing it off
to domainset_create. No functional change intended.

Differential Revision: https://reviews.freebsd.org/D46608
Reviewed by: markj