Add generic_signal() handler, which sets toys.signal and writes byte to toys.signalfd if set.
diff --git a/lib/lib.h b/lib/lib.h
index f8fcd8f..097e67d 100644
--- a/lib/lib.h
+++ b/lib/lib.h
@@ -174,6 +174,7 @@
 
 // signal
 
+void generic_signal(int signal);
 void sigatexit(void *handler);
 int sig_to_num(char *pidstr);
 char *num_to_sig(int sig);