Enable DTR when setting attributes.

In RXTX 2.1.7, `termios.c:tcsetattr(int, int, struct termios *)` sets `dcb.fDtrControl` to `DTR_CONTROL_ENABLE`; in 2.2pre2, this changed to `DTR_CONTROL_DISABLE`. It's hard to know why this  change was made without access to the RXTX Mailman archives, but I think it's a regression as consumers who previously expected to find DTR enabled “out of the box” don't get it any more. This problem is heightened by the fact that `gnu.io.SerialPort#setDTR(boolean)` seems to have no effect on Windows, so not only does existing code break, but there's no way to talk to devices which previously worked fine.

I've reverted the line and compiled new matching natives and this works fine for me, but I'm not sure this is the best approach globally as I don't understand why it would be bad to have DTS enabled in the first place. Perhaps the concern was that it might cause the connected device to start sending data before the library was ready to receive it?