fluxbox: add xsession profile

add .xsession profile to get fluxbox being started automatically
by XDM.

[Peter: Only install if not available in skeleton, use install -D,
	Make it an executable script and don't hide it]
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>