Issue 3251: set-default-paper-size bleeds over to subsequent file compilations

Issue 3251: set-default-paper-size bleeds over to subsequent file compilations

This patch lets set-default-paper-size copy $defaultpaper before
modifying it.  That way, restoring the original $defaultpaper at the
end of each processed file should also restore the paper dimensions.

[janek, 2013-03-17 16:30:44]

I like the description.  It's very clear.
LGTM
thanks!
Janek