setlayout: Allow setlayout(main) (#677, #652)

There is presently no way to revert to the 'default' (no) layout.
This patch makes setlayout(main) functionally equivalent to clearing
the applied layout (if present).

docs/keyd.scdoc

@@ -337,8 +337,8 @@ and not
 
 A layout is a special kind of layer intended for modifying alpha keys. Unlike
 layers, layouts cannot have any associated modifiers, and only one layout may
-be active at a given time. The current layout can be set using the _setlayout_
-action.
+be active at a given time. The default layout is called 'main', and can be
+changed using the _setlayout()_ action.
 
 For convenience, keyd ships with a number of common letter layouts in
 /usr/share/keyd/layouts. Before including these, it is instructive to inspect them.

src/config.c

@@ -674,7 +674,9 @@ static int parse_descriptor(char *s,
 						break;
 					case ARG_LAYOUT:
 						arg->idx = config_get_layer_index(config, argstr);
-						if (arg->idx == -1 || config->layers[arg->idx].type != LT_LAYOUT) {
+						if (arg->idx == -1 ||
+							(arg->idx != 0 && //Treat main as a valid layout
+							 config->layers[arg->idx].type != LT_LAYOUT)) {
 							err("%s is not a valid layout", argstr);
 							return -1;
 						}

src/keyboard.c

@@ -441,8 +441,11 @@ static void setlayout(struct keyboard *kbd, uint8_t idx)
 			kbd->layer_state[i].active = 0;
 	}
 
-	kbd->layer_state[idx].activation_time = 1;
-	kbd->layer_state[idx].active = 1;
+	// Setting the layout to main is equivalent to clearing all occluding layouts.
+	if (idx != 0) {
+		kbd->layer_state[idx].activation_time = 1;
+		kbd->layer_state[idx].active = 1;
+	}
 
 	kbd->output.on_layer_change(kbd, &kbd->config.layers[idx], 1);
 }