Make some similar functions use same parameter names with same constness

[khwilliamson]

An advantage of combining API elements that do almost the same thing into a single group, is that disparities in their signatures become much more visible. These are typically harmless, but sloppy practice. These two commits fix a couple of those cases sv_catpv and sv_catsv

[Leont]

SV * const dsv as argument type doesn't really make sense as an argument type. That const doesn't actually affect the interface.

[khwilliamson]

I checked, and there are about 400 instances in embed.fnc of the same type of declaration

[Leont]

I checked, and there are about 400 instances in embed.fnc of the same type of declaration

Fair. I don't like it but don't let that stop you from merging it.

[khwilliamson]

It can make things look "safer" than they are; I agree