setusercontext(): Set umask in a separate function, setclassumask()

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40686

(cherry picked from commit 0dd1705f584947625892988afe59e4cedb5cdd09)

Approved by: markj (mentor)