Don't recommend the -r flag to mountd.

It enables a non-standards compliant feature to support ancient
diskless terminals. We shouldn't recommend it without explanation.
(-r is currently the default in /etc/defaults/rc.conf along with -S,
this should likely be revisited.)

Instead, suggest setting mountd_enable. While not required to start
the NFS server, it is required for the "service mountd restart" command
below.

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D25760