New login_getcapenum(): Allows to read named enum values

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40684

(cherry picked from commit 90e914cd5ac1c8ecbf1ea88e9a65e7fa866c17a9)

Approved by: markj (mentor)