mkdir path specified by XDG_RUNTIME_DIR

to me it's strange to make sure directory exists if path is not specified by some environment variable and do nothing if it is.

@pwithnall outspeaked his concerns about this change:

Basically, there might be a reason why the dir is only created in one case here; and iirc it might not be spec-compliant to create it in some cases, because we can’t guarantee the directory will have the right behaviour or permissions, or be deleted once the session has finished.

but if he is right, then it should be commented that it is (not) done explicitly