Add -fen option

Add -fen option

A volatile string option -fen can now be used to set the initial position
of the initial variant through the command line, removing the need to
prepare a position file for this. Otherwise the effect should be the same
as when using -lpf: New games will be started from the specifoed  positon
until the New Variant dialog is used. The position is 'auto-sizing', i.e.
the FEN is used to set the board geometry as well.