core: do not filter out systemd.unit= and run-level specifier from kernel command...

core: do not filter out systemd.unit= and run-level specifier from kernel command line

Fixes a bug introduced by 846f1da465beda990c1c01346311393f485df467.

The commit 846f1da465beda990c1c01346311393f485df467 made systemd.unit=
filtered out from the command line. That causes debug-generator does not
work as expected on daemon-reexecute, and we cannot call `systemctl
daemon-reexecute` in our test suite running on nspawn.

Fixes issue reported in https://github.com/systemd/systemd/pull/23851#issuecomment-1170992052.