Replace magic 32 with REDIS_EVENTLOOP_FDSET_INCR

32 was the additional number of file descriptors Redis
would reserve when managing a too-low ulimit.  The
number 32 was in too many places statically, so now
we use a macro instead that looks more appropriate.

When Redis sets up the server event loop, it uses:
    server.maxclients+REDIS_EVENTLOOP_FDSET_INCR

So, when reserving file descriptors, it makes sense to
reserve at least REDIS_EVENTLOOP_FDSET_INCR FDs instead
of only 32.  Currently, REDIS_EVENTLOOP_FDSET_INCR is
set to 128 in redis.h.

Also, I replaced the static 128 in the while f < old loop
with REDIS_EVENTLOOP_FDSET_INCR as well, which results
in no change since it was already 128.

Impact: Users now need at least maxclients+128 as
their open file limit instead of maxclients+32 to obtain
actual "maxclients" number of clients.  Redis will carve
the extra REDIS_EVENTLOOP_FDSET_INCR file descriptors it
needs out of the "maxclients" range instead of failing
to start (unless the local ulimit -n is too low to accomidate
the request).

src/redis.c

@@ -1417,21 +1417,21 @@ void initServerConfig() {
 }
 
 /* This function will try to raise the max number of open files accordingly to
- * the configured max number of clients. It will also account for 32 additional
- * file descriptors as we need a few more for persistence, listening
- * sockets, log files and so forth.
+ * the configured max number of clients. It also reserves a number of file
+ * descriptors (REDIS_EVENTLOOP_FDSET_INCR) for extra operations of
+ * persistence, listening sockets, log files and so forth.
  *
  * If it will not be possible to set the limit accordingly to the configured
  * max number of clients, the function will do the reverse setting
  * server.maxclients to the value that we can actually handle. */
 void adjustOpenFilesLimit(void) {
-    rlim_t maxfiles = server.maxclients+32;
+    rlim_t maxfiles = server.maxclients+REDIS_EVENTLOOP_FDSET_INCR;
     struct rlimit limit;
 
     if (getrlimit(RLIMIT_NOFILE,&limit) == -1) {
         redisLog(REDIS_WARNING,"Unable to obtain the current NOFILE limit (%s), assuming 1024 and setting the max clients configuration accordingly.",
             strerror(errno));
-        server.maxclients = 1024-32;
+        server.maxclients = 1024-REDIS_EVENTLOOP_FDSET_INCR;
     } else {
         rlim_t oldlimit = limit.rlim_cur;
 
@@ -1445,11 +1445,11 @@ void adjustOpenFilesLimit(void) {
                 limit.rlim_cur = f;
                 limit.rlim_max = f;
                 if (setrlimit(RLIMIT_NOFILE,&limit) != -1) break;
-                f -= 128;
+                f -= REDIS_EVENTLOOP_FDSET_INCR;
             }
             if (f < oldlimit) f = oldlimit;
             if (f != maxfiles) {
-                server.maxclients = f-32;
+                server.maxclients = f-REDIS_EVENTLOOP_FDSET_INCR;
                 redisLog(REDIS_WARNING,"Unable to set the max number of files limit to %d (%s), setting the max clients configuration to %d.",
                     (int) maxfiles, strerror(errno), (int) server.maxclients);
             } else {