install: Perform usr-merge transition

Move all systemd units as well as the NSS plugin under /usr.

Note that this change is performed in the simplest possible way,
which has the downside of making the package no longer trivially
backportable to bookworm. This is considered fine since other
changes implemented during the trixie development cycle, namely
the 64-bit time_t transition, make such backports very hard to
perform.

Closes: #1064126
Thanks: Michael Biebl, Helmut Grohne