d/libnss-tls.nss: Install NSS service `tls` via dh_installnss (Closes #1024688)

Gioele Barabuccirequested to merge
gioele/nss-tls:use-dh-installnss into master

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.

It also removes the need to use Perl in postrm maintscripts. (Perl is a candidate for removal from essential, see https://wiki.debian.org/Proposals/EssentialOnDiet and grow-your-ideas#20.)

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