Remove doubleFork, handle SIGCHLD

This is a rather big change.  Rather than make spawned processes become
children of init, we handle them in xmonad.  As a side effect of this change,
we never need to use waitForProcess in any contrib module -- in fact, doing so
will raise an exception.  The main benefit to handling SIGCHLD is that xmonad
can now be started with 'exec', and will correctly clean up after inherited
child processes.

Main.hs

@@ -32,6 +32,7 @@ import qualified Properties
 -- for xmonad, and if it doesn't find one, just launches the default.
 main :: IO ()
 main = do
+    installSignalHandlers -- important to ignore SIGCHLD to avoid zombies
     args <- getArgs
     let launch = catchIO buildLaunch >> xmonad defaultConfig
     case args of