deb-build: ability to build without system dpkg.

use env var WITHOUT_DPKG=1 to enable it.
%bootstrap install now works using ar and tar.
%dpkg is redirected to %dpkgchroot
%remove is done manually.

Note that %dpkgchroot requires minimal libraries and obviously dpkg itself
to be installed in the chroot; if this is not the case then both it and
%dpkg (when using WITHOUT_DPKG) will fail. In this case, use %bootstrap
instead.
deb-build: fixes for manual removal.