Makefiles so that we can make changes to it centrally as needed and have
less mess. Fixes the sun2 build that needs rumpvfs after librump after
the latest changes.
XXX: the reverse mapping case (last subsubtest in t_basic) does
not make any sense, but apparently that how umapfs works. I'm not
familiar enough with the code to determine if this is a wanted
feature or a pure and simple bug.