fix (again) the handling of read errors in the debugger

Actually the read errors were doing what we wanted, but EOF was no
longer popping one debugger level.  The control transfer is a bit
gnarly, so explicitly grab the restart we might want to use and pass
it as an argument to DEBUG-READ.
delete ye olde FIXME relating to unbound variable warnings

Testing the code in the real system gives a full warning
fix sleep on most ratios

really really ensure that the second argument to nanosleep is an
integer
some tests of SLEEP with ratios
Fix sleep on ratios, avoiding consing.

Enable sleep tests for some platforms.