vty: Make vty_event dispatch signals and use it in the testcase

The testcase didn't work on Ubuntu 12.04 because vty_create will
directly call vty_event (e.g. not through the plt). This means
that the approach to override vty_event in the testcase failed.

Use the signal interface of libosmocore and make the testcase
use it. The signals can be generally useful as well.
diff --git a/include/osmocom/core/signal.h b/include/osmocom/core/signal.h
index b3a5aae..19a2688 100644
--- a/include/osmocom/core/signal.h
+++ b/include/osmocom/core/signal.h
@@ -19,6 +19,7 @@
 	SS_L_GLOBAL		= OSMO_SIGNAL_SS_RESERVED,
 	SS_L_INPUT,
 	SS_L_NS,
+	SS_L_VTY,
 };
 
 /* application-defined signal types. */