Updated paths to scan when importing zpool(s)

Added by-partlabel and by-partuuid to the default device search
path. Made made device names in by-label more preferable.

Signed-off-by: Thijs Cramer <thijs.cramer@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3892