stand: Make sure nobody has a NULL pointer for dv_cleanup

dv_cleanup is specified almost everywhere. Use nullsys instead of NULL
to indicate 'do nothing'. Also, be consistent in trailing commas that
were missing before.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D35913

(cherry picked from commit e98f952c82907ec39cebb4e66efd92691c835e05)