Final Remark: The easiest way to regain INT0 and INT1 is to solve the original problem.
This is caused only by RX0, in my tests of the serial port TX0 worked fine.
For a speed of 115.200 baud a simple RC group can filter the incoming signal up to a point where
RX0 is perfectly usable again. The RC filter consists of a 10k resistor in series and
100pf cap to ground.
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=107115
That's it, problem solved.
So I switched back from optiboot_atmega1284p-u1.hex to optiboot_atmega1284p-u0.hex...