MEDIUM: tune.http.maxhdr makes it possible to configure the maximum number of HTTP headers

For a long time, the max number of headers was taken as a part of the buffer
size. Since the header size can be configured at runtime, it does not make
much sense anymore.

Nothing was making it necessary to have a static value, so let's turn this into
a tunable with a default value of 101 which equals what was previously used.
diff --git a/src/proxy.c b/src/proxy.c
index d274dfe..caec45d 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -819,7 +819,7 @@
 		/* and now initialize the HTTP transaction state */
 		http_init_txn(s);
 
-		s->txn.hdr_idx.size = MAX_HTTP_HDR;
+		s->txn.hdr_idx.size = global.tune.max_http_hdr;
 		hdr_idx_init(&s->txn.hdr_idx);
 	}