repmgr: add parameter "shutdown_check_timeout"

Previously, "repmgr standby switchover" used the configuration file parameters
"reconnect_interval" and "reconnect_attempts" to define a timeout to determine
whether the current primary (demotion candidate) has shut down.

However, these parameters are intended for primary failure detection and are
generally lower in value, while a controlled shutdown may take longer, resulting
in the switchover being aborted as repmgr was not waiting long enough.

To prevent this happening, parameter "shutdown_check_timeout" has been added.
This complements the existing "standby_reconnect_timeout" parameter used
by "repmgr standby switchover".

Implements GitHub #504.

configfile.h

@@ -103,6 +103,7 @@ typedef struct
 	int			standby_follow_timeout;
 
 	/* standby switchover settings */
+	int			shutdown_check_timeout;
 	int			standby_reconnect_timeout;
 
 	/* node rejoin settings */
@@ -181,6 +182,7 @@ typedef struct
 		DEFAULT_PRIMARY_FOLLOW_TIMEOUT,	\
 		DEFAULT_STANDBY_FOLLOW_TIMEOUT,	\
 		/* standby switchover settings */ \
+		DEFAULT_SHUTDOWN_CHECK_TIMEOUT, \
 		DEFAULT_STANDBY_RECONNECT_TIMEOUT, \
 		/* node rejoin settings */ \
 		DEFAULT_NODE_REJOIN_TIMEOUT, \