Fix -var option with UCI_Chess960

Fix -var option with UCI_Chess960

The variant specified in the -var option would be overruled by the
"normal,fischerandom" emitted later under the influence of a UCI_Chess960
option. This is now solved by always sending the variants feature on 'uciok'.
The varList is now initialized to the -var string, and overwritten by the
protocol defaults before sending 'uci' only when it is still empty.
A UCI_Variants option then gets the chance to overwrite this, while
a UCI_Chess960 option will append "normal,fischeradom" to the list.