Fix https://github.com/lualatex/luaotfload/issues/297

Interword glue hasn’t been considered yet. This again adapts the
relevant logic from Context to our letterspacing method. The code is
deliberately simplistic and will most likely not address all
constellations of a glue preceding a character.

The Luatexbase API was consolidated. Since the priority_in_callback
functionality wasn’t used for anything but determining whether we
need to convert to an hlist first, there’s not really any gain in
keeping it around in the generic form.

This seems to help with the examples from #165 that rely on setting
``$OPENTYPEFONTS`` via the environment.

Referencing a non-existing value caused a branch to always pick Unix
path separators. The affair was pretty insidious since the paths we
received from kpse were sane so the behavior was only triggered with
manual overrides.