add a mkdir-p POST handler

add a mkdir-p POST handler

this adds a t=mkdir-p call to directories (accessed by their uri as
/uri/<URI>?t=mkdir=p&path=/some/path) which returns the uri for a
directory at a specified path before the given uri, regardless of
whether the directory exists or whether intermediate directories
need to be created to satisfy the request.

this is used by the migration code in MV to optimise the work of
path traversal which was other wise done on every file PUT