MINOR: global: Preset tune.max_http_hdr to its default value

By default, this tune parameter is set to MAX_HTTP_HDR. This assignment is done
after the configuration parsing, when we check the configuration validity. So
during the configuration parsing, its value is 0. Now, it is set to MAX_HTTP_HDR
from the start. So, it is possible to rely on it during the configuration
parsing.
diff --git a/src/haproxy.c b/src/haproxy.c
index dd0c3ec..2525613 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -164,6 +164,7 @@
 		.pattern_cache = DEFAULT_PAT_LRU_SIZE,
 		.pool_low_ratio  = 20,
 		.pool_high_ratio = 25,
+		.max_http_hdr = MAX_HTTP_HDR,
 #ifdef USE_OPENSSL
 		.sslcachesize = SSLCACHESIZE,
 #endif