use a prompt without colors for dumb terminals

for example using a terminal in the acme text editor.

this depends on the prompt theme "redhat" already existing, but it seems
to be bundled with zsh

.zshrc

@@ -2107,6 +2107,9 @@ fi
     else
         prompt grml
     fi
+    if [[ "$TERM" == dumb ]] ; then
+        prompt redhat
+    fi
 
 # Terminal-title wizardry