Normalise the config dir path (remove duplicate / or \ etc).

Fixes issue 3522: DMDirc breaks when using the XDG config dir.

Change-Id: I413f99de837f086246fa59210aabd1d98b2b1074
Reviewed-on: http://gerrit.dmdirc.com/500
Reviewed-by: Gregory Holmes <greboid@dmdirc.com>
Automatic-Compile: Gregory Holmes <greboid@dmdirc.com>

src/com/dmdirc/Main.java

                     configdir += fs + "DMDirc" + fs;
                 }
             }
+
+            configdir = new File(configdir).getAbsolutePath() + fs;
         }
 
         return configdir;