iptnl/ip6tnl: Unify iptunnel_print_help()

iptnl/ip6tnl: Unify iptunnel_print_help()

Reduce diff lines between iptnl and ip6tnl help printing code.

Use @struct link_util ->id field to print correct link help: all callers
now pass this data structure to iptunnel_print_help().

Get rid of custom print_usage() and usage() functions and use
iptunnel_print_help() directly, return from function on "... type
<help|garbage>" instead of exit(2).

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>