orig-source handler needs manual changes in the chroots

[ghost]

The orig-source handler can use apt-get source to download the upstream part of the package but this will fail on many buildd chroots which don't specify a deb-src location for apt.

Adding this here as a reminder to document this problem and possibly try to detect it in advance.

There are other issues (like the likely absence of fakeroot and the need for pbuilder inside the chroot) which may mean that it is useful to package a "check-my-chroots" script as part of the client setup process.