Whitelist SIGUSR1 to avoid auto-triggering errors.

This commit fixes issue #875 that was caused by the following events:

1) There is an active child doing BGSAVE.
2) flushall is called (or any other condition that makes Redis killing
the saving child process).
3) An error is sensed by Redis as the child exited with an error (killed
by a singal), that stops accepting write commands until a BGSAVE happens
to be executed with success.

Whitelisting SIGUSR1 and making sure Redis always uses this signal in
order to kill its own children fixes the issue.

src/rdb.c

@@ -1194,7 +1194,10 @@ void backgroundSaveDoneHandler(int exitcode, int bysignal) {
         redisLog(REDIS_WARNING,
             "Background saving terminated by signal %d", bysignal);
         rdbRemoveTempFile(server.rdb_child_pid);
-        server.lastbgsave_status = REDIS_ERR;
+        /* SIGUSR1 is whitelisted, so we have a way to kill a child without
+         * tirggering an error conditon. */
+        if (bysignal != SIGUSR1)
+            server.lastbgsave_status = REDIS_ERR;
     }
     server.rdb_child_pid = -1;
     server.rdb_save_time_last = time(NULL)-server.rdb_save_time_start;