MINOR: config: round up global.tune.bufsize to the next multiple of 2 void*

Since HTX casts the buffer to a struct and stores relative pointers at the
end, it is mandatory that its end is properly aligned. This patch enforces
a buffer size rounding up to the next multiple of two void*, thus 8 on
32-bit and 16 on 64-bit, to match what malloc() already does on the beginning
of the buffer. In pratice it will never be really noticeable since default
sizes already are such multiples.
diff --git a/src/haproxy.c b/src/haproxy.c
index 6c96794..4af8880 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -149,9 +149,9 @@
 		 }
 	},
 	.tune = {
-		.bufsize = BUFSIZE,
+		.bufsize = (BUFSIZE + 2*sizeof(void *) - 1) & -(2*sizeof(void *)),
 		.maxrewrite = -1,
-		.chksize = BUFSIZE,
+		.chksize = (BUFSIZE + 2*sizeof(void *) - 1) & -(2*sizeof(void *)),
 		.reserved_bufs = RESERVED_BUFS,
 		.pattern_cache = DEFAULT_PAT_LRU_SIZE,
 #ifdef USE_OPENSSL