Enable Salsa CI

This will help ensure easily machine detectable regressions don't slip into the code base.

This also makes any future contribution process faster and more reliable, as any contributor submitting a Merge Request will get immediate feedback, and the maintainers save time by not having to point out basic mistakes.

Additionally, includes fixes needed to make pipeline green:

piuparts

ERROR: FAIL: After purging files have been modified:
 /usr/sbin/unix_chkpwd	 owned by: libpam-modules-bin

lintian

E: libpam-modules-bin: bad-so-link-within-manual-page [usr/share/man/man5/environment.5.gz]

Reprotest is failing due to external reasons (salsa-ci-team/pipeline#376 (closed), https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1079175, broken archive-wide).