[letterspace] add Xetex compatibility for “letterspace”

When defining a font, Luaotfload interprets the value supplied to the
“letterspace” option as a percentage of the font size, mimicking Xetex
in yet another respect.
In order to preserve the former behavior (no scaling) a new option
**kernfactor** is introduced whose value is passed on to the
letterspacing feature unmodified.
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