lib: json_print: Add print_on_off()

lib: json_print: Add print_on_off()

The value of a number of booleans is shown as "on" and "off" in the plain
output, and as an actual boolean in JSON mode. Add a function that does
that.

RDMA tool already uses a function named print_on_off(). This function
always shows "on" and "off", even in JSON mode. Since there are probably
very few if any consumers of this interface at this point, migrate it to
the new central print_on_off() as well.

Signed-off-by: Petr Machata <me@pmachata.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David Ahern <dsahern@gmail.com>