Sentinel: test for writable config file.

This commit introduces a funciton called when Sentinel is ready for
normal operations to avoid putting Sentinel specific stuff in redis.c.

src/redis.h

@@ -1223,6 +1223,7 @@ void initSentinelConfig(void);
 void initSentinel(void);
 void sentinelTimer(void);
 char *sentinelHandleConfiguration(char **argv, int argc);
+void sentinelIsRunning(void);
 
 /* Scripting */
 void scriptingInit(void);