systemctl: don't confuse sysv code with generated units

systemctl: don't confuse sysv code with generated units

The SysV compat code checks whether there's a native unit file before looking
for a SysV init script. Since the newest rework generated units will show up in
the unit path, and hence the checks ended up assuming that there always was a
native unit file for each init script: the generated one.

With this change the generated unit file directory is suppressed from the
search path when this check is done, to avoid the confusion.