This reverts commit 9bc62cca21a0a171cbc220419af91b3d8f7e333b.
This stragegy didn't work. Our new gsubrs have not yet be defined
when the ice-9/boot-9.scm code is loaded.
xyzzy_open_file (nee make_xyzzy) was not stripping off the magic
prefix before searching, and was thus always creating a new entry in
the map and returning a "" string.