store preprocessed font names with db
scan fontconfig dirs even though ``$OSFONTDIR`` is set

This is essentially what this change to the TL2011 version does:
    https://github.com/lualatex/luaotfload/commit/31530badb2681ff3423fe31f5eccf3d05fab1956#L2R657
in response to this issue:
    https://github.com/lualatex/luaotfload/issues/11