Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Factor out the code shared between NFS client and server into its own
module. With r203732 it became apparent that creating the sysctl nodes
twice causes at least a warning, however the whole code shouldn't be
present twice in the first place.

Discussed with: rmacklem