export: fix usage with rootless containers

Fix usage of export when rootless containers are used without vfs.  We
join the conmon process namespaces as the container is running in a
different one.

There can be a problem if the user specify a different path for the
conmon process, and then the file is deleted.  In this case podman
won't be able to find the conmon process to join.

Closes: https://github.com/containers/libpod/issues/2027

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

cmd/podman/main.go

@@ -30,6 +30,7 @@ var cmdsNotRequiringRootless = map[string]bool{
 	"version": true,
 	"create":  true,
 	"exec":    true,
+	"export":  true,
 	// `info` must be executed in an user namespace.
 	// If this change, please also update libpod.refreshRootless()
 	"login":   true,