[merge] Control-backspace to drop into the debugger

[Martin Pool]

This patch lets you press C-\ to go into pdb at any time.   This is
pretty useful if it has hung, or just if you want to see what is
taking time.

As always you can then type 'c' to continue execution.  This will in
some cases find problems where we don't handle EINTR correctly, but
those are bugs in other cases and should be fixed.

-- 
Martin
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