obexd: AuthorizePush: Support to return folder name for file storage

obexd: AuthorizePush: Support to return folder name for file storage

The obex agent usually returns the full path by getting the default
folder and filename from the Filename property of the transfer object
which is not convenient.

The patch helps that the obex agent can return the folder name suffixed
with '/' or new full path or even null which will use the default name
if new_name is NULL and the default folder if the new_folder is NULL in
opp_chkput().