Add generic_signal() handler, which sets toys.signal and writes byte to toys.signalfd if set.
diff --git a/main.c b/main.c
index 92ec89f..38f6d1a 100644
--- a/main.c
+++ b/main.c
@@ -80,6 +80,7 @@
   }
   toys.old_umask = umask(0);
   if (!(which->flags & TOYFLAG_UMASK)) umask(toys.old_umask);
+  toys.signalfd--;
 }
 
 // Setup toybox global state for this command.