prioritize ``prefmodifiers`` oer ``subfamily``

follow-up to these previous modifications:

* https://github.com/phi-gamma/luaotfload/commit/fcdbafd0c86fdba08
* https://github.com/phi-gamma/luaotfload/commit/2938e7d63404f1d2e
* https://github.com/phi-gamma/luaotfload/commit/0e51cff12b24d264b

addresses https://github.com/lualatex/luaotfload/issues/108
i.e. "name:DejaVu Serif" does not resolve to the Condensed face anymore.