Extend the fullscreen command

Rather than just toggling the fullscreen modes, allow to set them
directly with:

    fullscreen enable|toggle [global]
    fullscreen disable

For compatibility, retain the previous command and its toggling behavior:

    fullscreen [global]

fixes #1120

include/con.h

@@ -172,6 +172,18 @@ void con_fix_percent(Con *con);
  */
 void con_toggle_fullscreen(Con *con, int fullscreen_mode);
 
+/**
+ * Enables fullscreen mode for the given container, if necessary.
+ *
+ */
+void con_enable_fullscreen(Con *con, fullscreen_mode_t fullscreen_mode);
+
+/**
+ * Disables fullscreen mode for the given container, if necessary.
+ *
+ */
+void con_disable_fullscreen(Con *con);
+
 /**
  * Moves the given container to the currently focused container on the given
  * workspace.