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.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #16217

lib/libspl/include/statcommon.h

@@ -39,5 +39,7 @@
 void print_timestamp(uint_t);
 /* Return timestamp in either Unix or standard format in provided buffer */
 void get_timestamp(uint_t, char *, int);
+/* convert time_t to standard format */
+void format_timestamp(time_t, char *, int);
 
 #endif /* _STATCOMMON_H */