Change default key-bindings of mark commands

*Shift-m* feels much more natural for *mark_range* and *Ctrl-u* is decent
enough for *unmark_all*.

config.def.h

@@ -90,9 +90,9 @@ static const keymap_t keys[] = {
 	{ 0,            XK_minus,         g_zoom,               -1 },
 	{ 0,            XK_KP_Subtract,   g_zoom,               -1 },
 	{ 0,            XK_m,             g_toggle_image_mark,  None },
-	{ 0,            XK_M,             g_reverse_marks,      None },
-	{ ControlMask,  XK_M,             g_mark_range,         None },
-	{ ControlMask,  XK_m,             g_unmark_all,         None },
+	{ 0,            XK_M,             g_mark_range,         None },
+	{ ControlMask,  XK_m,             g_reverse_marks,      None },
+	{ ControlMask,  XK_u,             g_unmark_all,         None },
 	{ 0,            XK_N,             g_navigate_marked,    +1 },
 	{ 0,            XK_P,             g_navigate_marked,    -1 },
 	{ 0,            XK_braceleft,     g_change_gamma,       -1 },