d/libnss-mdns.nss: Install NSS services `mdns*` via dh_installnss (Closes: #1009646)

dh_installnss provides a declarative way to install NSS services.

In comparison to the existing scripts, dh_installnss also handles DPKG_ROOT and various edge-cases such as invalid /etc/nsswitch.conf files, database lines with comments, etc.

In addition, the code in postinst that deals with reordering the hosts services can be dropped because it no longer run since old-stable (Debian 10).