homework: allow specifying a dir component in CIFS services

homework: allow specifying a dir component in CIFS services

Allow specifying CIFS services in the format //host/service/subdir/… to
allow multiple homedirs on the same share, and not in the main dir of
the share.

All other backends allow placing the data store at arbitrary places,
let's allow this too for the CIFS backend. This is particularly useful
for testing.