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/haproxy.c b/src/haproxy.c
index 20b18f8..a8d0fad 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2986,7 +2986,7 @@
 			if (global.cpu_map.proc[relative_pid-1])
 				global.cpu_map.thread[relative_pid-1][i] &= global.cpu_map.proc[relative_pid-1];
 
-			if (i < LONGBITS &&       /* only the first 32/64 threads may be pinned */
+			if (i < MAX_THREADS &&       /* only the first 32/64 threads may be pinned */
 			    global.cpu_map.thread[relative_pid-1][i]) {/* only do this if the thread has a THREAD map */
 #if defined(__FreeBSD__) || defined(__NetBSD__)
 				cpuset_t cpuset;