udev: use the usual set of load paths for udev rules

udev: use the usual set of load paths for udev rules

This adds /usr/local/lib/udev/rules.d to the search path on non-split-usr systems.
On split-usr systems, the paths with /usr/-prefixes are added too.
In the past, on split-usr systems, it made sense to only load rules from
/lib/udev/rules.d, because /usr could be mounted late. But we don't support running
without /usr since 80758717a63, so in practice it doesn't matter whether the
rules files are in /lib/udev/rules.d or /usr/lib/udev/rules.d. Distributions
that maintain the illusion of functional split-usr are welcome to simply not put any
files in /usr/lib/udev/rules.d/.

In practice this doesn't change much, but it makes udev more consistent with the
rest of the systemd suite.