refactoring of engineoutput

refactoring of engineoutput

This refactoring patch separates the back-end part out of
xengineoutput.c, and puts it in a separate file engineoutput.c.
A new header engineoutput.h included by both defines the routines
for cross calling. Similarly wengineo.c will be stripped from its
back-end code, (and renamed to wengineoutput.c), so WinBoard and
XBoard now both use engineoutput.c rather than maintaining duplicate
code in their front-end part.

I also let the back-end now call EngineOutputUpdate directly, rather then
through a relay in the front-end, by renaming it to SetProgramStats
(the former name of the relay in winboard.c / xboard.c).

Moved some templates of functions in engineoutput.c from winboard.h
to frontend.h