Fix zed io-spare.sh dash incompatibility

The zed's io-spare.sh script defines a vdev_status() function to query
the 'zpool status' output for obtaining the status of a specified vdev.
This function contains a small awk script that uses a parameter
expansion (${parameter/pattern/string}) supported in bash but not
in dash.  Under dash, this fails with a "Bad substitution" error.

This commit replaces the awk script with a (hopefully more portable)
sed script that has been tested under both bash and dash.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2536