Add the ability to disable the current symlinks.

Check packages in the alternatives and stalled_pkg_timeout lists in
the configuration file and Conf.pm.  Make sure they are current and
valid.

sbuild-createchroot should import most of buildd-make-chroot.
(note: only needed for buildd use--accounts and passwords needed for access)

sbuild-createchroot should update schroot config
(need /etc/schroot/conf.d)

sbuild-createchroot should update the package reference list
(sbuild-checkpackages --set)

Provide a way of generating current reference lists from debootstrapped chroots.
Make sbuild-createchroot create an initial reference list on chroot creation.

Split utility programs into library functions. Functions should operate on
chroot objects, or plain files where appropriate.

Log.pm: Handle nolog case by always opening LOG and PLOG.

Log.pm: Log functions should be generic (writeout of sbuild version/build info
and symlink creation should be customisable).

avg-pkg-build-* should use the database inside the chroot.  Add options to
specify the chroot and database to read.  Rename to sbuild-stats.
