fix invocation of duplicate handlers for merge-generics

fix invocation of duplicate handlers for merge-generics

* libguile/modules.c (resolve_duplicate_binding): Fix unbound -> #f
  conversion for the imported bindings.  Pass the existing entry in the
  import obarray as the resolved var (7th arg), and properly pass #f as
  the value (8th arg) if there is no such binding.  Fixes
  merge-generics; before, the <boolean> type test (indicating no
  previous value) was not being triggered.  This bug has been present
  since 2007 at least, though it was not in 1.8.

* test-suite/tests/modules.test ("duplicate bindings"): Add a test that
  the var and val are both #f.  These types are used by GOOPS.