system: add new subcommand "migrate"

it is useful to migrate existing containers to a new version of
podman.  Currently, it is needed to migrate rootless containers that
were created with podman <= 1.2 to a newer version which requires all
containers to be running in the same user namespace.

Closes: https://github.com/containers/libpod/issues/2935

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

cmd/podman/commands.go

@@ -77,6 +77,7 @@ func getSystemSubCommands() []*cobra.Command {
 		_pruneSystemCommand,
 		_renumberCommand,
 		_dfSystemCommand,
+		_migrateCommand,
 	}
 }