[variable-search] Correctly implement on OS X.

On Mac OS X, we just want to pass RTLD_DEFAULT to dlsym() rather
than a handle from dlopen() (which doesn't work).
[variable-search] Reorganize to share more code.
[variable-search] Add tests.
Merge pull request #784 from waywardmonkeys/variable-search-improvements

Variable search improvements
[dfmc-execution] Restore dfmc-execution from history.
[dfmc-execution] Don't use functional-dylan.
[dfmc-execution] Remove references to timer primitives.

These primitives no longer exist.
[dfmc-execution] Clean up whitespace.
[dfmc-execution] not-found -> unfound.
[dfmc-execution] Fix types with load-dll.

This takes a symbol, not a string now.