Use expected zpool_status_t type

Both the zpool_import_status() and zpool_get_status() functions
return the zpool_status_t enum. This explicit type should be used
rather than the more generic int type.

This patch makes no functional change and should only be considered
code cleanup. It happens to have been done in the context of #2094
because that's when I noticed this issue.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Richard Yao <ryao@gentoo.or
Issue #2094