fix try-module-autoload, which did not detect failure to find the file

fix try-module-autoload, which did not detect failure to find the file

* libguile/load.c (scm_primitive_load_path): If the second argument is a
  procedure, call it like a thunk.

* doc/ref/api-evaluation.texi (Load Paths): Update docs.

* module/ice-9/boot-9.scm (resolve-interface): Use `unless'.
  (try-module-autoload): Use the new primitive-load-path to detect
  failure to find an appropriate file.  Fixes a bug reported by Diogo
  F. S. Ramos.  Thanks to Noah Lavine for tracking it down.