Use unsigned values for termios setting

This allows the last (most significant) bit to be set without
overflow errors. The error becomes visible after adding the
CRTSCTS constant on linux/386:

   const tcCRTSCTS uint = 0x80000000

serial_unix.go

@@ -143,7 +143,7 @@ func setTermSettingsBaudrate(speed int, settings *syscall.Termios) error {
 		return &PortError{code: InvalidSpeed}
 	}
 	// revert old baudrate
-	BAUDMASK := 0
+	var BAUDMASK uint
 	for _, rate := range baudrateMap {
 		BAUDMASK |= rate
 	}