made parentDir return a Maybe FilePath; removed most uses of it

parentDir is less safe than takeDirectory, especially when working
with relative FilePaths. It's really only useful in loops that
want to terminate at /

This commit was sponsored by Audric SCHILTKNECHT.

Command/Fix.hs

@@ -43,7 +43,7 @@ perform file link = do
 			<$> getSymbolicLinkStatus file
 #endif
 #endif
-		createDirectoryIfMissing True (parentDir file)
+		createDirectoryIfMissing True (takeDirectory file)
 		removeFile file
 		createSymbolicLink link file
 #ifdef WITH_CLIBS