JSON output support for zpool list

This commit adds support for zpool list command to output the list of
ZFS pools in JSON format using '-j' option.. Information about available
pools is collected in nvlist which is later printed to stdout in JSON
format.

Existing options for zfs list command work with '-j' flag. man page for
zpool list is updated accordingly.

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

Author: Umer Saleem