rc.d/mountd: Require /etc/exports only when there is no other exports file

[christos]

Summary

mountd exits if it cannot read any exports files. Requiring /etc/exports
unconditionally stops mountd from starting on a server that exports ZFS
datasets alone (/etc/zfs/exports). Require /etc/exports only when it is
the only exports file.

MFC after: 1 week

[rmacklem]

I suppose this is ok. It would be nice to note
somewhere that /etc/exports is always needed
for NFSv4. (A /etc/zfs/exports only setup can
be done for NFSv3 only.)

[rmacklem]

In D60044#1377567, @rmacklem wrote:

I suppose this is ok. It would be nice to note
somewhere that /etc/exports is always needed
for NFSv4. (A /etc/zfs/exports only setup can
be done for NFSv3 only.)

It might be better to require /etc/exports when either
nfsv4_server_enable or nfsv4_server_only are "yes".

[christos]

In D60044#1377730, @rmacklem wrote:

In D60044#1377567, @rmacklem wrote:

I suppose this is ok. It would be nice to note
somewhere that /etc/exports is always needed
for NFSv4. (A /etc/zfs/exports only setup can
be done for NFSv3 only.)

It might be better to require /etc/exports when either
nfsv4_server_enable or nfsv4_server_only are "yes".

I'm not really experienced with NFS, so we could get more people to weigh in on this. @markj what do you think?