path-util: make default PATH match global ALT defaults
This one is easier to diff against systemd-stable v254.10 than against
the previous commit.
The global ALT defaults as of filesystem >= 3 are:
* sbin directories are omitted for user services;
* every sbin directory, if present, is ordered right before its matching
bin.
* /usr/local/* are ordered last (I do not condone this, but if we agree
to fix this, fix it everywhere at once);
* executable locations in /usr precede or replace executable locations in /.
We already conflict with filesystem < 3 due to different changes.