Add unshare to podman

This command lets the user run a command in a new user namespace like `unshare -u`.
It uses the implementation of unshare in buildah. ( fixes #1388 )

Signed-off-by: Divyansh Kamboj <kambojdivyansh2000@gmail.com>

cmd/podman/commands.go

@@ -20,6 +20,7 @@ func getMainCommands() []*cobra.Command {
 		_refreshCommand,
 		_searchCommand,
 		_statsCommand,
+		_unshareCommand,
 	}
 
 	if len(_varlinkCommand.Use) > 0 {