mountd startup: enable NFSv4 if needed on restart

The mountd script in rc.d sets vfs.nfsd.server_max_nfsvers correctly
when it is run at system startup, relying on the kernel default.
However, if NFSv4 was enabled in /etc/rc.conf later, and the script
was re-run to restart mountd, the sysctl was still set to 3.
Set the sysctl to the right value in all cases.

Reviewed by: rmacklem
MFC after: 1 week