JSON output support zfs mount

This commit adds support for zfs mount to display mounted file systems
in JSON format using '-j' option. Data is collected in nvlist which is
printed in JSON format. man page for zfs mount 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