Cwd::fast_abs_path's untaint should allow for multiline directories

Cwd::fast_abs_path's untaint should allow for multiline directories

This bug was noticed via https://github.com/dagolden/file-chdir/issues/3 and testing has led to this being the cause.
The problem is worse on some platforms (notably cygwin in this case) when abs_path is implemented by fast_abs_path.
Since File::chdir tests for proper behavior when a directory contains a newline, this bug then breaks File::chdir (one of my favorites and very useful xplatform tool).

Yes this should have tests, but since it will involve creating a directory with a newline, I thought I would do better to leave that to someone with better knowledge than I.