Accept URLs of form file:/home... also.

--- a/src/Pure/General/url.ML	Thu Nov 23 20:34:21 2006 +0100
+++ b/src/Pure/General/url.ML	Thu Nov 23 22:14:26 2006 +0100
@@ -65,6 +65,7 @@
   Scan.this_string "file:///" |-- scan_path_root >> File ||
   Scan.this_string "file://localhost/" |-- scan_path_root >> File ||
   Scan.this_string "file://" |-- scan_host -- scan_path >> RemoteFile ||
+  Scan.this_string "file:/" |-- scan_path_root >> File ||
   Scan.this_string "http://" |-- scan_host -- scan_path >> Http ||
   Scan.this_string "ftp://" |-- scan_host -- scan_path >> Ftp;