spawn: Create sessions for processes

Each process spawned from awesome now gets its own session and process group.
This makes sure they aren't connected to awesome in any way any more. This
especially fixes some problems with signals.

Signed-off-by: Uli Schlachter <psychon@znc.in>

spawn.c

@@ -268,6 +268,7 @@ spawn_child_callback(gpointer user_data)
 
     sigemptyset(&empty);
     sigprocmask(SIG_SETMASK, &empty, NULL);
+    setsid();
 }
 
 /** Spawn a command.