Invert logic for the last commit

This makes it more clear that the option is meant to be a special
case (it *disables* part of the focus handling). Also, when
initializing the config data structure with zeros, it will get
initialized with the right value.

Furthermore, the config file parser now also accepts various values
which represent "true", not only numbers.

include/config.h

@@ -77,7 +77,11 @@ struct Config {
         int container_stack_limit;
         int container_stack_limit_value;
 
-        bool focus_follows_mouse;
+        /** By default, focus follows mouse. If the user explicitly wants to
+         * turn this off (and instead rely only on the keyboard for changing
+         * focus), we allow him to do this with this relatively special option.
+         * It is not planned to add any different focus models. */
+        bool disable_focus_follows_mouse;
 
         const char *default_border;