Added ashift support to "zpool add".

This adds the "-o property=value" construct to zpool add.
It also retrieves the default configured ashift value from the zpool
and uses that value if no specific ashift value has been given.
Fixed an assertion failure when looking for exported pools.
MacZFS doesn't support devids, so don't try to read these.
Changed "zpool iostat" to use the full terminal width, if needed.