kernel-install: support full set of config files and drop-ins

kernel-install: support full set of config files and drop-ins

This brings the handling of config for kernel-install in line with most of
systemd, i.e. we search the set of paths for the main config file, and the full
set of drop-in paths for drop-ins.

This mirrors what 07f5e35fe7967c824a87f18a3a1d3c22e5be70f5 did for udev.conf.
That change worked out fine, so I hope this one will too.

The update in the man page is minimal. I think we should split out a separate
page for the config file later on.

One motivating use case is to allow a drop-in to be created for temporary
config overrides and then removed after the operation is done.