debian: Modernize use of dh_install.

debian: Modernize use of dh_install.

Originally I intended this as just a cleanup, but as a side effect it also
installs some files from the install tree in debian/tmp instead of from
_debian.  This should avoid a reported problem in which ovs-bugtool was
being created in the source directory instead of the build directory (I
still don't see why this happened).

Reported-by: Sébastien RICCIO <sr@swisscenter.com>
Tested-by: Sébastien RICCIO <sr@swisscenter.com>
Acked-by: Simon Horman <horms@verge.net.au>
CC: Simon Horman <horms@verge.net.au>