0.9.2.18: various error &co reporting improvements and build tweaks

0.9.2.18: various error &co reporting improvements and build tweaks
 * print the cell name for CELL-ERRORs when *PRINT-ESCAPE* is true.
 * print just the filename, not the SHARED-OBJECT structure for failed
   LOAD-SHARED-OBJECT.
 * improve our Darwin dlerror(3)-shim reporting; also clean up warnings from
   ppc-darwin-dlsim.c and reindent it for 80 columns.
 * move reporting of failed contribs to make-target-contrib.sh, and re-add
   --disable-debugger, which had accidentally fallen off.