Ensure exit is possible

If shutdown has been initiated (via maxadmin or by sending a
SIGINT or SIGTERM) and a SIGTERM is received, the process
is terminated.

If shutdown has been initiated (via maxadmin or by sending a
SIGINT or SIGTERM) and a SIGINT (Ctrl-C) is received, a warning
is printed that shutdown is in progess. Then, if an additional
SIGINT is received, the process is terminated.

So, in practice:
- If MaxScale is running as a daemon, the first SIGTERM initiates
  shutdown and a second one unconditionally terminates the process.
- If MaxScale is running in the console, the first Ctrl-C initiates
  shutdown, the second prints a warning and the third terminates
  the process.

This is to ensure that MaxScale can be forced to exit, in
case some thread is hung for whatever reason, and is thus preventing
the controlled shutdown.

include/maxscale/maxscale.h

@@ -44,4 +44,14 @@ void maxscale_reset_starttime(void);
 time_t maxscale_started(void);
 int maxscale_uptime(void);
 
+/**
+ * Initiate shutdown of MaxScale.
+ *
+ * This functions informs all threads that they should stop the
+ * processing and exit.
+ *
+ * @return How many times maxscale_shutdown() has been called.
+ */
+int maxscale_shutdown(void);
+
 MXS_END_DECLS