MINOR: threads: add a MAX_THREADS define instead of LONGBITS

This one allows not to inflate some structures when threads are
disabled. Now struct global is 1.4 kB instead of 33 kB.

Should be backported to 1.8 for ease of backporting of upcoming
patches.
diff --git a/src/listener.c b/src/listener.c
index dfd36cc..fd70726 100644
--- a/src/listener.c
+++ b/src/listener.c
@@ -963,7 +963,7 @@
 
 	conf->bind_proc |= proc;
 	if (thread) {
-		for (i = 0; i < LONGBITS; i++)
+		for (i = 0; i < MAX_THREADS; i++)
 			if (!proc || (proc & (1UL << i)))
 				conf->bind_thread[i] |= thread;
 	}