old one (the one renamed to wstrconcat). The new wstrappend(dst, src)
will modify and return dst, without creating a new string to hold the
result, except if dst==NULL, in which case its equivalent to calling
wstrdup(src)
be sure to replace wstrappend() with wstrconcat() anywhere in your code
because a new wstrappend() function will be implemented that will have
different semantics and if your code will use the new one instead of
the old will break.
