OtherCommands: re-add the "volume" command

This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients.  On request, I
add it to the protocol again.

src/command/AllCommands.cxx

@@ -161,6 +161,7 @@ static const struct command commands[] = {
 	{ "unsubscribe", PERMISSION_READ, 1, 1, handle_unsubscribe },
 	{ "update", PERMISSION_CONTROL, 0, 1, handle_update },
 	{ "urlhandlers", PERMISSION_READ, 0, 0, handle_urlhandlers },
+	{ "volume", PERMISSION_CONTROL, 1, 1, handle_volume },
 };
 
 static const unsigned num_commands = sizeof(commands) / sizeof(commands[0]);

src/command/OtherCommands.cxx

@@ -236,6 +236,39 @@ handle_setvol(Client &client, gcc_unused int argc, char *argv[])
 	return CommandResult::OK;
 }
 
+CommandResult
+handle_volume(Client &client, gcc_unused int argc, char *argv[])
+{
+	int relative;
+	if (!check_int(client, &relative, argv[1]))
+		return CommandResult::ERROR;
+
+	if (relative < -100 || relative > 100) {
+		command_error(client, ACK_ERROR_ARG, "Invalid volume value");
+		return CommandResult::ERROR;
+	}
+
+	const int old_volume = volume_level_get();
+	if (old_volume < 0) {
+		command_error(client, ACK_ERROR_SYSTEM, "No mixer");
+		return CommandResult::ERROR;
+	}
+
+	int new_volume = old_volume + relative;
+	if (new_volume < 0)
+		new_volume = 0;
+	else if (new_volume > 100)
+		new_volume = 100;
+
+	if (new_volume != old_volume && !volume_level_change(new_volume)) {
+		command_error(client, ACK_ERROR_SYSTEM,
+			      "problems setting volume");
+		return CommandResult::ERROR;
+	}
+
+	return CommandResult::OK;
+}
+
 CommandResult
 handle_stats(Client &client,
 	     gcc_unused int argc, gcc_unused char *argv[])

src/command/OtherCommands.hxx

@@ -51,6 +51,9 @@ handle_rescan(Client &client, int argc, char *argv[]);
 CommandResult
 handle_setvol(Client &client, int argc, char *argv[]);
 
+CommandResult
+handle_volume(Client &client, int argc, char *argv[]);
+
 CommandResult
 handle_stats(Client &client, int argc, char *argv[]);