RFR: DIO-17 UART.generateBreak() is not implemented correctly

[boris kvartskhava]

issue: https://bugs.openjdk.java.net/browse/DIO-17
webrev: http://cr.openjdk.java.net/~onazarkina/DIO-17/

This change resolves an issue with invalid DIO implementation
    of UART.generateBreak(...) method.
   The result of the call to ioctl(..., TCSBRKP, positive) is undefined.
    Instead, an approach with the sequency is taken:
    ioctl(..., TIOCSBRK, 0);
    followed by usleep() followed by
    ioctl(..., TIOCCBRK, 0);