Don't restart during upgrade (#853016)

Simon McVittierequested to merge
smcv/nodm:no-restart-853016 into master

Restarting a display manager during upgrade causes the GUI session to be terminated, with potential for data loss.

This appears to have regressed in 0.13-3 when moving from debhelper compat level 9 to 12, due to dh_installinit and dh_installsystemd behaviour changes. See #853016.

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