unmask async exceptions after fork

Turns out that forkProcess masks async exceptions. Unmask them so that the
daemon code can use them for thread IPC.

There is some risk this introduces breakage in git-annex, but it would be
breakage that would already occur when the assistant was run with
--foreground.

Utility/Daemon.hs

@@ -16,6 +16,7 @@ import Utility.LogFile
 
 #ifndef mingw32_HOST_OS
 import System.Posix
+import Control.Concurrent.Async
 #else
 import System.PosixCompat
 #endif
@@ -46,7 +47,9 @@ daemonize logfd pidfile changedirectory a = do
 		nullfd <- openFd "/dev/null" ReadOnly Nothing defaultFileFlags
 		redir nullfd stdInput
 		redirLog logfd
-		a
+		{- forkProcess masks async exceptions; unmask them inside
+		 - the action. -}
+		wait =<< asyncWithUnmask (\unmask -> unmask a)
 		out
 	out = exitImmediately ExitSuccess
 #else