Call exportfs -v once for NFS shares.

Call exportfs -v once for NFS shares.

At the moment we call exportfs -v every time we check whether an
NFS share is active. This happens every time you run a zfs or
zpool command, making them extremely slow when you have a lot of
exports. The time taken is approx O(n2) of the number of shares.

This commit stores the output from exportfs -v in a temporary file
and use this to speed up subsequent accesses.

This mechanism is still too slow - if you have tens of thousands
of NFS shares it will still be painful running ANY zfs/zpool
command.

Signed-off-by: Gunnar Beutner <gunnar@beutner.name>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #341