Changes in pl73

Two new command bk  and eco
bk to provide list of moves from the book in form:
	move response relative-value

eco provides classification of the current opening
	listing options available from the current position

gnuan - new interactive interface provides the possibility
	to look at new lines or alternative moves
	ability to process PGN games

Improved book

ability to load xboard fen positions

New improved Makefile, easier to configure.

All known bug fixes applied to the search engine.
