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.