Bug#531611: Invalid master alternatives, please tell us

[Raphael Hertzog]

Hello,

since dpkg 1.15, update-alternatives is more strict and forbids lots of
silly operations that were previously accepted. Unfortunately improper
usage of u-a in the past could lead to cruft files that cause bugs on
upgrade. You have examples in #530633 and #531611.

| Setting up gnome-session (2.26.1-6) ...
| Installing new version of config file /etc/gnome/defaults.list ...
| update-alternatives: error: alternative x-session-manager.1.gz can't be slave of  x-session-manager: it is a master alternative.

Since alternatives are shared among numerous packages, it makes sense
sometimes to do the corresponding cleanup directly in dpkg to avoid
having to duplicate it in all the packages implementing the alternatives.
I've done so for the two cases listed above but if you encounter other
case where it makes sense to fix it in dpkg, please let us know.

Cheers,
-- 
Raphaël Hertzog

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