Revert "extract system user … creation into a separate script":

In the discussion around it, ebourg indicated that he prefers
the inline code over a separate script, if it needs to be present
at all (which IMHO is necessary).

I kept the “do nothing if the user exists” check first, though;
there’s no need to try to create it over and over on every upgrade.

This reverts commit 036902fe
and c8966d77.