Terminating the plymouth/console agents when the wall agent takes over
can happen asynchronously, after all the pw queries are async anyway and
hence can be seen by both the plymouth/console agents and the wall
agent.
By stopping the two agents with "--no-block" we add a bit of robustness,
since trouble of them exiting won't block the wall agent to start.
This addresses the issue the previous commit fixes in a different way.
This makes things a bit simpler and the build a bit faster, because we don't
have to rewrite files to do the trivial substitution. @rootbindir@ is always in
our internal $PATH that we use for non-absolute paths, so there should be no
functional change.
