Readd zpool_clear_label() from OpenSolaris

This patch restores the zpool_clear_label() function from
OpenSolaris. This was removed by commit d603ed6 because
it wasn't clear we had a use for it in ZoL. However, this
functionality is a prerequisite for adding the 'zpool labelclear'
command from FreeBSD.

As part of bringing this change in the zpool_clear_label()
function was changed to use fstat64_blk(2) for compatibility
with Linux.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1126