Output pair kerning according to sf->preferred_kerning.

This addresses some additional issues raised in #1929.

The main changes are as follows.

* Support sf->preferred_kerning in S. F. D..
* Allow for over-riding this value in the Generate Fonts dialogue.
* Map all kerns from U. F. O. native files (which lack script designators) to the DFLT script.
* Add disabled code for guessing scripts.