latter would lead to undefined symbols if DDB not defined.
(2) check for break on console, and therefore debugger entry (if ddb
in kernel) earlier, so that the device doesn't need to be open.
(3) return immediately after breaking into the debugger in comeint().
(4) only do the normal character input routine in comintr if receive
mask yeilds _EXACLTY_ LSR_RXRDY. if there's only a receive
error, or there's a receive error _and_ a received character,
do comeint().
(former two by me. latter two from Bob Baron <rvb@cs.cmu.edu>.)
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