Added "fullscreen" group option to force a window to open fullscreen.

This change will add a "fullscreen" group option (includes update of the manpage). This forces a window to be initially fullscreen. I implemented this for use with Rollemup Pinball, which for some reason did not run fullscreen under JWM, though it does so under other window managers.

If you want to try it yourself, the game is available from http://www.pro-linux.de/files/rollemup/Rollemup.tar.gz (i386 binaries). It requires the rather ancient libstdc++.so.2.8 (e.g. available in the [compat-libstdc++ package of Fedora 3] (http://ftp.pbone.net/mirror/archive.fedoraproject.org/fedora/linux/core/3/i386/os/Fedora/RPMS/compat-libstdc++-8-3.3.4.2.i386.rpm)). Before running the game it's best to run `xrandr -s 640x480`; the game does not set the screen resolution itself.