AOF: don't abort on write errors unless fsync is 'always'.

A system similar to the RDB write error handling is used, in which when
we can't write to the AOF file, writes are no longer accepted until we
are able to write again.

For fsync == always we still abort on errors since there is currently no
easy way to avoid replying with success to the user otherwise, and this
would violate the contract with the user of only acknowledging data
already secured on disk.

src/redis.h

@@ -663,6 +663,8 @@ struct redisServer {
     int aof_lastbgrewrite_status;   /* REDIS_OK or REDIS_ERR */
     unsigned long aof_delayed_fsync;  /* delayed AOF fsync() counter */
     int aof_rewrite_incremental_fsync;/* fsync incrementally while rewriting? */
+    int aof_last_write_status;      /* REDIS_OK or REDIS_ERR */
+    int aof_last_write_errno;       /* Valid if aof_last_write_status is ERR */
     /* RDB persistence */
     long long dirty;                /* Changes to DB from the last save */
     long long dirty_before_bgsave;  /* Used to restore dirty on failed BGSAVE */