Refactor putuserrec()

This started with a Coverity issue (CID 33230) which got me looking at this function and wondering: why is str2 being NUL-terminated here? Why is the length of str2 to be calculated on successive lines? What is with this (long)((long)((long)((long)) typecast?

This was some of the oldest code in Synchronet (along with a lot of the other functions in this file). I tried to keep as much intact as possible while still improving the logic and readability.