Fix maxclients error handling

Everywhere in the Redis code base, maxclients is treated
as an int with (int)maxclients or `maxclients = atoi(source)`,
so let's make maxclients an int.

This fixes a bug where someone could specify a negative maxclients
on startup and it would work (as well as set maxclients very high)
because:

    unsigned int maxclients;
    char *update = "-300";
    maxclients = atoi(update);
    if (maxclients < 1) goto fail;

But, (maxclients < 1) can only catch the case when maxclients
is exactly 0.  maxclients happily sets itself to -300, which isn't
-300, but rather 4294966996, which isn't < 1, so... everything
"worked."

maxclients config parsing checks for the case of < 1, but maxclients
CONFIG SET parsing was checking for case of < 0 (allowing
maxclients to be set to 0).  CONFIG SET parsing is now updated to
match config parsing of < 1.

It's tempting to add a MINIMUM_CLIENTS define, but... I didn't.

These changes were inspired by antirez#356, but this doesn't
fix that issue.

src/config.c

@@ -600,7 +600,7 @@ void configSetCommand(redisClient *c) {
     } else if (!strcasecmp(c->argv[2]->ptr,"maxclients")) {
         int orig_value = server.maxclients;
 
-        if (getLongLongFromObject(o,&ll) == REDIS_ERR || ll < 0) goto badfmt;
+        if (getLongLongFromObject(o,&ll) == REDIS_ERR || ll < 1) goto badfmt;
 
         /* Try to check if the OS is capable of supporting so many FDs. */
         server.maxclients = ll;

src/redis.h

@@ -786,7 +786,7 @@ struct redisServer {
     list *clients_waiting_acks;         /* Clients waiting in WAIT command. */
     int get_ack_from_slaves;            /* If true we send REPLCONF GETACK. */
     /* Limits */
-    unsigned int maxclients;        /* Max number of simultaneous clients */
+    int maxclients;                 /* Max number of simultaneous clients */
     unsigned long long maxmemory;   /* Max number of memory bytes to use */
     int maxmemory_policy;           /* Policy for key eviction */
     int maxmemory_samples;          /* Pricision of random sampling */