dgit: Do not execute END blocks in children.

END blocks run inside children created by fork or piped open (the
latter, if they run perl code), on exit or (I think maybe) die.  This
is far from appropriate.

So far symptoms of this bug seem to be limited to duplicated error
messages but I have not done a thorough analysis.  Closes:#850052.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>