samples, tests, boards: Switch main return type from void to int

As both C and C++ standards require applications running under an OS to
return 'int', adapt that for Zephyr to align with those standard. This also
eliminates errors when building with clang when not using -ffreestanding,
and reduces the need for compiler flags to silence warnings for both clang
and gcc.

Most of these changes were automated using coccinelle with the following
script:

@@
@@
- void
+ int
main(...) {
	...
-	return;
+	return 0;
	...
}

Approximately 40 files had to be edited by hand as coccinelle was unable to
fix them.

Signed-off-by: Keith Packard <keithp@keithp.com>

samples/bluetooth/central_ht/src/main.c

@@ -263,14 +263,14 @@ BT_CONN_CB_DEFINE(conn_callbacks) = {
 	.disconnected = disconnected,
 };
 
-void main(void)
+int main(void)
 {
 	int err;
 
 	err = bt_enable(NULL);
 	if (err) {
 		printk("Bluetooth init failed (err %d)\n", err);
-		return;
+		return 0;
 	}
 
 	printk("Bluetooth initialized\n");
@@ -279,8 +279,9 @@ void main(void)
 
 	if (err) {
 		printk("Scanning failed to start (err %d)\n", err);
-		return;
+		return 0;
 	}
 
 	printk("Scanning successfully started\n");
+	return 0;
 }