fix Issue 13413 - dmd does not follow symlink when searching for dmd.conf

- first search argv0 in PATH then resolve symbolic links

[Issue 13413](https://issues.dlang.org/show_bug.cgi?id=13413 "dmd does not follow symlink when searching for dmd.conf")
Since this use of `filename` does not survive the { }, it's best to declare a new variable name.