*-login: Add client_vfuncs.free() that is called when client refcount=0

This can be useful for plugins that want to run something after proxying
ends.

Use an empty default function so plugins can call super.free() without
having to check if it's NULL.

src/imap-login/imap-login-client.c

@@ -786,6 +786,7 @@ static struct client_vfuncs imap_client_vfuncs = {
 	imap_proxy_get_state,
 	client_common_send_raw_data,
 	imap_client_input_next_cmd,
+	client_common_default_free,
 };
 
 static const struct login_binary imap_login_binary = {

src/imap-urlauth/imap-urlauth-login.c

@@ -176,6 +176,7 @@ static struct client_vfuncs imap_urlauth_vfuncs = {
 	NULL,
 	client_common_send_raw_data,
 	NULL,
+	client_common_default_free,
 };
 
 static const struct login_binary imap_urlauth_login_binary = {

src/login-common/client-common.c

@@ -335,6 +335,9 @@ bool client_unref(struct client **_client)
 	i_assert(client->destroyed);
 	i_assert(client->login_proxy == NULL);
 
+	if (client->v.free != NULL)
+		client->v.free(client);
+
 	if (client->ssl_proxy != NULL)
 		ssl_proxy_free(&client->ssl_proxy);
 	if (client->input != NULL)
@@ -359,6 +362,10 @@ bool client_unref(struct client **_client)
 	return FALSE;
 }
 
+void client_common_default_free(struct client *client ATTR_UNUSED)
+{
+}
+
 void client_destroy_oldest(void)
 {
 	struct client *client;

src/login-common/client-common.h

@@ -123,6 +123,7 @@ struct client_vfuncs {
 	void (*send_raw_data)(struct client *client,
 			      const void *data, size_t size);
 	bool (*input_next_cmd)(struct client *client);
+	void (*free)(struct client *client);
 };
 
 struct client {
@@ -273,6 +274,7 @@ void client_send_raw_data(struct client *client, const void *data, size_t size);
 void client_send_raw(struct client *client, const char *data);
 void client_common_send_raw_data(struct client *client,
 				 const void *data, size_t size);
+void client_common_default_free(struct client *client);
 
 void client_set_auth_waiting(struct client *client);
 void client_auth_send_challenge(struct client *client, const char *data);

src/pop3-login/client.c

@@ -334,6 +334,7 @@ static struct client_vfuncs pop3_client_vfuncs = {
 	pop3_proxy_get_state,
 	client_common_send_raw_data,
 	pop3_client_input_next_cmd,
+	client_common_default_free,
 };
 
 static const struct login_binary pop3_login_binary = {