Added a command-line key generation tool. Currently builds and runs

on Linux, but the (very few) platform-specific bits are already
abstracted out of the main code, so it should port to other
platforms with a minimum of fuss.

[originally from svn r3762]

unix/uxcons.c

@@ -313,3 +313,8 @@ void frontend_keypress(void *handle)
      */
     return;
 }
+
+int is_interactive(void)
+{
+    return isatty(0);
+}