fs: Prevent invalid directory links

A file with a name like "gemini:example" would previously result in the
following invalid link:

    => gemini:example gemini:example

Fix by prepending a "./" before each filename, so that the resulting
link looks like:

    => ./gemini:example gemini:example

fs.go

@@ -169,7 +169,7 @@ func dirList(w ResponseWriter, f fs.File) {
 		}
 		link := LineLink{
 			Name: name,
-			URL:  (&url.URL{Path: name}).EscapedPath(),
+			URL:  "./" + url.PathEscape(name),
 		}
 		fmt.Fprintln(w, link.String())
 	}