kern_domainset: Split domainset validation logic into a separate function

[bnovkov]

Summary

This patch 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.