setusercontext(): Move priority setting in new setclasspriority()

In preparation for setting priorities from '~/.login_conf' and to ease
reading of setusercontext().

No functional change.

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40350

(cherry picked from commit 7b94ec550ef6e7b75d608e125e9b78478197d4e7)

Approved by: markj (mentor)