Tag alias: gunpoint -> at_gunpoint

Aliasing gunpoint to at_gunpoint.

I noticed the gunpoint tag needed some cleaning. I assume it's supposed to mean "A character is being threatened with a gun," but some people are confusing it with gun_to_head (held by the same character) and some others are confusing it with pov_aiming. I've cleaned these cases up, but I think the tag should be aliased to at_gunpoint to prevent confusion.

One point. Gun to head is used for situations where a gun is pointed directly to a head for any reason (self imposed or otherwise).

Gunpoint is obviously a broader and possibly different area. I could get behind excluding self-inflicted situations. It's probably not typical to describe someone as holding themselves "at gunpoint".

Did the original alias.