Allow setting NFS server scope and owner.

By default NFS server reports as scope and owner major the host UUID
value and zero for owner minor. It works good in case of standalone
server. But in case of CARP-based HA cluster failover the values
should remain persistent, otherwise some clients like VMware ESXi
get confused by the change and fail to reconnect automatically.

The patch makes server scope, major owner and minor owner values
configurable via sysctls. If not set (by default) the host UUID
value is still used.

Reviewed by: rmacklem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D31952

(cherry picked from commit 272c4a4dc5fbe3d82d735c5b9a3b6faab052808b)