path-lookup: downgrade again the messages where we log for units

This partially reverts 7ad94c716d6403233d04c4d37cb14df958c9b65d.

After that commit commands such as "systemctl enable" and friends
printed the search path information multiple times in its output, which
is ugly.

If we want the search paths to be printed at a higher log level, then we
should do this in PID 1 only, i.e. split the printing out of the normal
path lookup logic and invoke that explicitly from PID 1 but not in the
auxiliary tools.