unescape \n \r etc in string

[Krux02]

I have a macro with a string argument. The string is partially preserved, but 
all n from the string are now litterally n and t. so how do I unescape 
thesevalues to create a string object out of them? All my ideas right now feel 
like a lot of work with some uncertenty if I did everything correct, so I 
thought I might better ask, even though the implementation is propably trivial 
I do not think that the correct implementation is.