rc.conf(5): add <service>_umask to run the service using this value

None of tools working with login classes change umask(1)
and we had no ways to specify non-default umask for a service
not touching its startup script. This change makes in possible.

Some file-sharing services that create new files may benefit from it.

Differential: https://reviews.freebsd.org/D36309

(cherry picked from commit 160a2f2cdda806e104c0d3194bfb84b208ad1ae8)