rc.d: Fix mountd service script.

[des]

Summary

This script references variables beloning to the nfsd and zfs services,
therefore it needs to load their configurations.

MFC after: 1 week

[rmacklem]

I don't see a problem with this, but I am not conversant
with the rc scripting stuff.

All the variables used by mountd are set in /usr/src/libexec/rc/rc.conf
and will be updated by the system's /etc/rc.conf file(s).
As such, I thought the values for all those variables were available to
the script?

I'll resign as a reviewer, since I do not know the rc script stuff, although
I'll note the change looks fine to me.

[des]

load_rc_config always loads /etc/rc.conf but does not load everything under /etc/rc.conf.d, so if one service needs to inspect configuration variables belonging to another, it must explicitly load_rc_config that other service in case those variables are set in /etc/rc.conf.d.