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
To indicate that the there're no more entries, these wrappers return false but
did leave the passed pointed unmodified.
However EOF is not an error and is a very common case so initialize the output
argument to NULL even in this case so callers don't need to do that.
Fixes: #8721