webapi: use t=mkdir-with-children instead of a children= arg to t=mkdir .

webapi: use t=mkdir-with-children instead of a children= arg to t=mkdir .

This is safer: in the earlier API, an old webapi server would silently ignore
the initial children, and clients trying to set them would have to fetch the
newly-created directory to discover the incompatibility. In the new API,
clients using t=mkdir-with-children against an old webapi server will get a
clear error.