podman-registry: simpler, safer invocations

First: fix podman-registry script so it preserves the initial $PODMAN,
so all subsequent invocations of ps, logs, and stop will use the
same binary and arguments. Until now we've handled this by requiring
that our caller manage $PODMAN (and keep it the same), but that's
just wrong.

Next, simplify the golang interface: move the $PODMAN setting into
registry.go, instead of requiring e2e callers to set it. (This
could use some work: the local/remote conditional is icky).

IMPORTANT: To prevent registry.go from using the wrong podman binary,
the Start() call is gone. Only StartWithOptions() is valid now.

And, minor cleanup: comments, and add an actual error-message check

Reason for this PR is a recurring flake, #18355, whose multiple
failure modes I truly can't understand. I don't think this PR
is going to fix it, but this is still necessary work.

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/e2e/libpod_suite_remote_test.go

@@ -115,7 +115,7 @@ func (p *PodmanTestIntegration) StopRemoteService() {
 	}
 }
 
-// MakeOptions assembles all the podman main options
+// getRemoteOptions assembles all the podman main options
 func getRemoteOptions(p *PodmanTestIntegration, args []string) []string {
 	networkDir := p.NetworkConfigDir
 	podmanOptions := strings.Split(fmt.Sprintf("--root %s --runroot %s --runtime %s --conmon %s --network-config-dir %s --network-backend %s --cgroup-manager %s --tmpdir %s --events-backend %s --db-backend %s",