Some distros install nologin as /usr/sbin/nologin, others as
/sbin/nologin.
Since we can't really on merged-usr everywhere (where the path wouldn't
matter), make the path build time configurable via -Dnologin-path=.
Closes#13028
This PR implements the first part of RFE #8046. I.e. this allows to
write:
```
u username -:300
```
Where the uid is chosen automatically but the gid is fixed.