login.conf(5): umask has no default value

The umask is simply left unchanged if no explicit value is specified in
the login class capabilities database.

PR: 271747
Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40345

(cherry picked from commit 0d1fe948d97d2d6e3708349b74cf438f9604a8d1)

Approved by: markj (mentor)