d/libnss-sss.nss: Install NSS service `sss` via dh_installnss
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.
The other commits in this MR fix a couple of other minor libnss-sss packaging issues.