Alias carrying_on_back -> piggyback

BUR #4698 has been approved by @evazion.

create alias carrying_on_back -> piggyback

I'm not sure if the tag should be renamed to help ESL people.

-1; piggybacks aren't the only method of carrying someone on your back. The phrase carrying_on_back could also refer to a fireman's_carry, making it ambiguous and therefore a bad alias.

I'd say fireman's carry is more akin to "carrying on shoulders"?

The bulk update request #4698 (forum #180739) has been approved by @evazion.

Well, we're treating piggyback as a subtag of carrying, so it makes sense to have a carrying_on_something utility alias for it, and carrying on back seems like the best option.