Treat EOF same as quit command

Treat EOF same as quit command

A EOF during reading from the GUI caused UCI2WB to exit through a different
path as a 'quit' command. Now the EOF detection just fakes a 'quit -1'
command was received, and then processes that as usual. This means it
also causes proper termination of the engine when it occurs during thinking
(sending 'stop'). The '-1' appendage to the command allows the 'quit'
handling to return an error code.