setclassumask(): Accept 'inherit' as a value

'inherit' explicitly indicates that the umask should not be changed.

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Relnotes: yes
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40687

(cherry picked from commit c328e6c6ccaa4cdf921c16d68a2f2c5992dd3b72)

Approved by: markj (mentor)