systemd-path: add 'system-search-configuration'

systemd-path: add 'system-search-configuration'

This provides functionality to replace what was provided by the preceding
revert:

  $ build/systemd-path system-search-configuration --suffix=sysctl.d
  /etc/sysctl.d:/run/sysctl.d:/usr/local/lib/sysctl.d:/usr/lib/sysctl.d

The result is identical, but more generic, since by changing suffix we can also
get the answer for sysusers.d, tmpfiles.d, and any other of the directories
which follow the same general rule.