use linux style options for config file and add a system config file for XBoard

use linux style options for config file and add a system config file for XBoard

This patch makes XBoard write the options to its conf file in
Linux style, "-option value" rather than Windows style "/option=value".

The WinBoard option parsing code has always understood both formats
(including any hybrids), but the format it preferred for writing might
look quite strange to Linux users. I also attached a file to be installed
as /etc/xboard/xboard.conf on "make install".