Add --unsetenv & --unsetenv-all to remove def environment variables

Podman adds a few environment variables by default, and
currently there is no way to get rid of them from your container.
This option will allow  you to specify which defaults you don't
want.

--unsetenv-all will remove all default environment variables.

Default environment variables can come from podman builtin,
containers.conf or from the container image.

Fixes: https://github.com/containers/podman/issues/11836

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

libpod/container_internal_linux.go

@@ -709,18 +709,6 @@ func (c *Container) generateSpec(ctx context.Context) (*spec.Spec, error) {
 		g.AddAnnotation(annotations.ContainerManager, annotations.ContainerManagerLibpod)
 	}
 
-	// Only add container environment variable if not already present
-	foundContainerEnv := false
-	for _, env := range g.Config.Process.Env {
-		if strings.HasPrefix(env, "container=") {
-			foundContainerEnv = true
-			break
-		}
-	}
-	if !foundContainerEnv {
-		g.AddProcessEnv("container", "libpod")
-	}
-
 	cgroupPath, err := c.getOCICgroupPath()
 	if err != nil {
 		return nil, err