Try and clean up the multiple formal loading support a bit, based on

Try and clean up the multiple formal loading support a bit, based on
suggestions from Greg Lehey some time ago.  In the face of multiple
potential file formats, try and give a more sensible error than just
ENOEXEC.

XXX a good case can be made that the loading process is wrong - the linker
should locate the file first (using the search paths etc), then run the
loaders to see if they recognize it.  While the present system allows for
the possibility of different search paths for different formats, we do not
use it and it just makes things more complicated than they need to be.