Fix infinite loop on startup if ulimit too low

Fun fact: rlim_t is an unsigned long long on all platforms.

Continually subtracting from a rlim_t makes it get smaller
and smaller until it wraps, then you're up to 2^64-1.

This was causing an infinite loop on Redis startup if
your ulimit was extremely (almost comically) low.

The case of (f > oldlimit) would never be met in a case like:

    f = 150
    while (f > 20) f -= 128

Since f is unsigned, it can't go negative and would
take on values of:

    Iteration 1: 150 - 128 => 22
    Iteration 2:  22 - 128 => 18446744073709551510
    Iterations 3-∞: ...

To catch the wraparound, we use the previous value of f
stored in limit.rlimit_cur.  If we subtract from f and
get a larger number than the value it had previously,
we print an error and exit since we don't have enough
file descriptors to help the user at this point.

Thanks to @bs3g for the inspiration to fix this problem.
Patches existed from @bs3g at antirez#1227, but I needed to repair a few other
parts of Redis simultaneously, so I didn't get a chance to use them.

src/redis.c

@@ -1519,6 +1519,17 @@ void adjustOpenFilesLimit(void) {
                 limit.rlim_max = f;
                 if (setrlimit(RLIMIT_NOFILE,&limit) != -1) break;
                 f -= REDIS_EVENTLOOP_FDSET_INCR;
+                if (f > limit.rlim_cur) {
+                    /* Instead of getting smaller, f just got bigger.
+                     * That means it wrapped around its unsigned floor
+                     * and is now closer to 2^64.  We can't help anymore. */
+                    redisLog(REDIS_WARNING,"Failed to set max file limit. "
+                        "You requested maxclients of %d "
+                        "but your 'ulimit -n' is set to %llu. "
+                        "Please increase your 'ulimit -n' to at least %llu.",
+                        server.maxclients, oldlimit, maxfiles);
+                    exit(1);
+                }
             }
             if (f < oldlimit) f = oldlimit;
             if (f != maxfiles) {