Remove an unused local variable
Make Wels*Snprintf return values be non-negative

This makes sure the windows version of these functions behave
more like the posix version. The posix *snprintf returns how
much would have been written if the buffer had been large
enough, which we don't know easily in the windows versions.

This basically means that we can assume that the return value is
>= 0 now, which can simplify the calling code.
Simplify the use of return values from Wels*Snprintf

Also similarly simplify the use of the return value from
WelsStrftime and GetCodeName/GetLibName/GetVerNum.
Merge pull request #724 from mstorsjo/string-cleanup

Unify return values from Wels*Snprintf and simplify calling code