Improve `zpool labelclear`

Improve `zpool labelclear`

1) As implemented the `zpool labelclear` command overwrites
the calculated offsets of all four vdev labels even when only a
single valid label is found.  If the device as been re-purposed
but still contains a valid label this can result in space no
longer owned by ZFS being zeroed.  Prevent this by verifying
every label removed is intact before it's overwritten.

2) Address a small bug in zpool_do_labelclear() which prevented
labelclear from working on file vdevs.  Only block devices support
BLKFLSBUF, try the ioctl() but when it's reported as unsupported
this should not be fatal.

3) Fix `zpool labelclear` so it can be run on vdevs which were
removed from the pool with `zpool remove`.  Additionally, allow
intact but partial labels to be cleared as in the case of a failed
`zpool attach` or `zpool replace`.

4) Remove LABELCLEAR and LABELREAD variables for test cases.

Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8500
Closes #8373
Closes #6261