install, remove, install same version: nsswitch.conf bad

if i install libnss-mdns (via avahi-daemon via cups, say), then *deinstall* it, then *reinstall* it, /etc/nsswitch.conf's host: line doesn't get the mdns entry added (back). (or, so it appears to me.)

i'm guessing that on the second add, libnss-mdns.postinst is being called with "configure 0.10-7", or some such, which causes the postinst script to decide it's being re-installed and doesn't need to modify /etc/nsswitch.conf. unfortunately, the intervening remove removed that initial changes to /etc/nsswitch.conf.

the symptoms, in my case was CUPS complaining "Unable to locate printer "BRN-015165.local".

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: libnss-mdns 0.10-7
ProcVersionSignature: Ubuntu 4.4.0-72.93-generic 4.4.49
Uname: Linux 4.4.0-72-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CurrentDesktop: LXDE
Date: Thu Apr 13 18:20:41 2017
InstallationDate: Installed on 2017-04-10 (3 days ago)
InstallationMedia: Lubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160720)
SourcePackage: nss-mdns
UpgradeStatus: No upgrade log present (probably fresh install)