Fix zpool set feature@<feature>=disabled

Commit e4010f2 accidentally allows zpool to set pool features to
"disabled"; this should only be allowed at pool creation. This commit
adds additional checks and test coverage to 'zpool set'.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7402