Add enclosure_symlinks option to vdev_id

Add an 'enclosure_symlinks' option to vdev_id.conf. This creates
consistently named symlinks to the enclosure devices (/dev/sg*) based
off the configuration in vdev_id.conf. The enclosure symlinks show
up in /dev/by-enclosure/<prefix>-<channel><num>. The links make it
make it easy to run sg_ses on a particular enclosure device. The
enclosure links are created in addition to the normal
/dev/disk/by-vdev links.

'enclosure_symlinks' is only valid in sas_direct configurations.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Simon Guest <simon.guest@tesujimath.org>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #8194