Make zfs-share service resilient to stale exports

The are a few cases where stale entries in /etc/exports.d/zfs.exports
will cause the nfs-server service to fail when starting up.

Since the nfs-server startup consumes /etc/exports.d/zfs.exports, the
zfs-share service (which rebuilds the list of zfs exports) should run
before the nfs-server service.

To make the zfs-share service resilient to stale exports, this change
truncates the zfs config file as part of the zfs share -a operation.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #13775