Tag every log line with role.

Every log contains, just after the pid, a single character that provides
information about the role of an instance:

S - Slave
M - Master
C - Writing child
X - Sentinel

src/redis.h

@@ -619,6 +619,7 @@ struct clusterState;
 
 struct redisServer {
     /* General */
+    pid_t pid;                  /* Main process pid. */
     char *configfile;           /* Absolute config file path, or NULL */
     int hz;                     /* serverCron() calls frequency in hertz */
     redisDb *db;