setclasspriority(): New possible value 'inherit'

It indicates to the login.conf machinery (setusercontext() /
setclasscontext()) to leave priority alone, effectively inheriting it
from the parent process.

PR: 271749
Reviewed by: emaste, yuripv
Approved by: emaste (mentor)
MFC after: 3 days
Relnotes: yes
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40690

(cherry picked from commit d162d7e2ad327ceada4102c5d6779f9ecd809502)

Approved by: markj (mentor)