Fix exec icon location and bring it into XDG compliance

diff --git a/mountconfig/fuser.py b/mountconfig/fuser.py
index 74eaafc..3071885 100644
--- a/mountconfig/fuser.py
+++ b/mountconfig/fuser.py
@@ -163,7 +163,7 @@
 
     def readPixmaps(self):
         self.pix = {
-            "exec": UserIcon("exec"),
+            "exec": UserIcon("application-x-executable"),
             "owner": UserIcon("user"),
             "pid": UserIcon("tux"),
             "file": UserIcon("file")}