Module loading working on FreeBSD (and unbreak).

(1) Commit eec525c43adade97ff94d839810faf7cb35bd87f broke module
    support, because we *do* still need some exported variable
    definitions, as for a module to actually work, we need the
    per-module _INCLUDE/_LIBS settings.

(2) FreeBSD's nsdispatch() will leave dlerror() returning a complaint
    about "_nss_cache_cycle_prevention_function"; we need to clear the
    error state before the dlsym() call, so that any error found
    afterwards must have come from that dlsym() call.  Fix is just an
    extra call to dlerror(), which should be portable.

I can now use sqlite3 from a module, in FreeBSD.