Add DesktopNames=CDE to cde.desktop

Suggestion from Giacomo Comes <comes@naic.edu>:

    In this way, after a login, the desktop manager will set the
    environment variables: XDG_CURRENT_DESKTOP and XDG_SESSION_DESKTOP

cde/contrib/desktopentry/cde.desktop

@@ -1,5 +1,6 @@
 [Desktop Entry]
 Name=CDE
+DesktopNames=CDE
 Comment=Use this session to boot into the Common Desktop Environment
 Keywords=Common Desktop Environment
 Exec=/usr/dt/bin/Xsession