bin/mdw-build, bin/mdw-sbuild, bin/mdw-sbuild-server: Additional packages.

bin/mdw-build, bin/mdw-sbuild, bin/mdw-sbuild-server: Additional packages.

Support supplying additional packages to the builder, in
`dist-BUILD.pkgs'.  Release builds don't do this, for obvious reasons.

There's an annoying fiddle where we have to separate out the supplied
packages into per-architecture sets because sbuild is too stupid to do
this for itself and ends up using the wrong architecture.