serial consoles to the 1.2 branch. Includes:
* dynamically allocated new-config softc
* remove dependencies on Decstation CPU type, use flags in softc instead.
* pass in values for softc flags (q.v.) from parent-specific attach code
* #ifdef out rts/cts flow control support until it's known to work on
200s.
* redo initialization of serial consoles (needs more work).
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