use nanosleep instead of usleep

SUSv3 says that:
	The useconds argument shall be less than one million. If the value of
	useconds is 0, then the call has no effect.
and actually NetBSD's implementation rejects such a value with EINVAL.
use nanosleep which has no such a limitation instead.

src/debug.c

@@ -294,8 +294,11 @@ void debugCommand(redisClient *c) {
     } else if (!strcasecmp(c->argv[1]->ptr,"sleep") && c->argc == 3) {
         double dtime = strtod(c->argv[2]->ptr,NULL);
         long long utime = dtime*1000000;
+        struct timespec tv;
 
-        usleep(utime);
+        tv.tv_sec = utime / 1000000;
+        tv.tv_nsec = (utime % 1000000) * 1000;
+        nanosleep(&tv, NULL);
         addReply(c,shared.ok);
     } else {
         addReplyError(c,