add handling for parity and character size config.

add handling for parity and character size config.

i wanted to talk modbus to a thing using a uchcom rs485 adapter,
but i needed even parity enabled to do that which the code didnt
support. this pulls in the necessary changes from netbsd uchcom.c
r1.26. it does not pull in the reset changes in 1.26 because netbsd
r1.28 reverts the reset code back to what we have now.

existing functionality tested by felix kronlage-dammers
ok patrick@