spell check on unix should be ctrl-shift-k like seamonkey.

Thanks to Stephen Walker for the patch

mail/components/compose/content/messengercompose.xul

@@ -184,7 +184,12 @@
   <key keycode="&findPrevCmd.key2;"  command="cmd_findPrev" modifiers="shift"/>
   
   <!-- Options Menu -->
-  <key id="key_checkspelling" key="&checkSpellingCmd.key;" command="cmd_spelling"  modifiers="accel"/>
+#ifdef XP_UNIX
+  <key id="key_checkspelling" key="&checkSpellingCmd.key;" command="cmd_spelling" modifiers="accel, shift"/>
+#else
+  <key id="key_checkspelling" key="&checkSpellingCmd.key;" command="cmd_spelling" modifiers="accel"/>
+#endif
+
 #ifdef XP_WIN
   <key keycode="&checkSpellingCmd.key2;" command="cmd_spelling"/>
 #endif