Try path without sbin even if compiled with split-bin=true

Try path without sbin even if compiled with split-bin=true

I'm working on the transition to merged sbin in Fedora. While the transition is
happening (and probably for a while after), we need to compile systemd with
split-bin=true to support systems upgraded from previous versions. But when the
system has been upgraded and already has /usr/sbin that is a symlink, be nice
and give $PATH without sbin.

We check for both /usr/sbin and /usr/local/sbin. If either exists and is not a
symlink to ./bin, we retain previous behaviour. This means that if both are
converted, we get the same behaviour as split-bin=false, and otherwise we
get the same behaviour as before.

sd-path uses the same logic. This is not a hot path, so I got rid of the nulstr
macros that duplicated the logic.