[Freeciv-Dev] (PR#13136) extending the unit class

["Jason Dorje Short" <jdorje@xxxxxxxxx>]

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=13136 >

In unittype.h there is a unit class, along with some accessor code for it.

This enum is about 5 years old.  However nobody uses it (except some AI
code which is likely buggy).  I would like to fix it up as a superset of
the move-class concept.  Ultimately the move-class may be replaced and
only the unit-class will remain.  In the meantime there is a fair bit of
confusion because class names overlap ("Air" is a unit-class and a
move-class name).

Ultimately the unit-classes should be defined in the ruleset.

-jason