JSON output support for zpool status

This commit adds support for zpool status command to displpay status
of ZFS pools in JSON format using '-j' option. Status information is
collected in nvlist which is later dumped on stdout in JSON format.
Existing options for zpool status work with '-j' flag. man page for
zpool status is updated accordingly.

Signed-off-by: Umer Saleem <[email protected]>

Author: Umer Saleem