[xmonad] Patch: improved support for replacing an existing window manager

Hi there! First patch from Bluetile's repository for consideration to go into xmonad mainline. This is the original patch (see http://osdir.com/ml/xmonad@haskell.org/2009-05/msg00010.html) plus two more patches, which * make this take effect only if the '--replace' flag is given * add a config option 'alwaysReplace' to override the --replace flag Best regards! Jan