Skip the range 0-254 when allocating attributes.

We can no longer trick the font loader into allocating attributes our way, thus we need some level of damage control. It will consist of:

1) These commits, avoiding conflicts between the integer ranges where attributes are handled.

2) Code (yet uncommited) in luaotfload that moves attributes defined by the font loader to the canonical table in luatexbase.attributes.

NB: luatexbase-attr uses ``tex.setattribute`` which, though it exists (``ltexlib.c``), is not mentioned in the docs -- I’m having a gut feeling that it is deprecated somehow and could be replaced by directly writing to ``tex.attributes``.