rootless: enable linger if /run/user/UID not exists

at least on Fedora 30 it creates the /run/user/UID directory for the
user logged in via ssh.

This needs to be done very early so that every other check when we
create the default configuration file will point to the correct
location.

Closes: https://github.com/containers/libpod/issues/3410

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/rootless/rootless_unsupported.go

@@ -29,6 +29,12 @@ func GetRootlessGID() int {
 	return -1
 }
 
+// EnableLinger configures the system to not kill the user processes once the session
+// terminates
+func EnableLinger() (string, error) {
+	return "", nil
+}
+
 // TryJoinFromFilePaths attempts to join the namespaces of the pid files in paths.
 // This is useful when there are already running containers and we
 // don't have a pause process yet.  We can use the paths to the conmon