BUG/MINOR: checks: don't send proxy protocol with agent checks

James Brown reported that agent-check mistakenly sends the proxy
protocol header when it's configured. This is obviously wrong as
the agent is an independant servie and not a traffic port, let's
disable this.

This fix must be backported to 1.7 and possibly 1.6.
diff --git a/src/checks.c b/src/checks.c
index 778fc6a..bee7101 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1559,7 +1559,7 @@
 	ret = SF_ERR_INTERNAL;
 	if (proto->connect)
 		ret = proto->connect(conn, check->type, quickack ? 2 : 0);
-	if (s->check.send_proxy) {
+	if (s->check.send_proxy && !(check->state & CHK_ST_AGENT)) {
 		conn->send_proxy_ofs = 1;
 		conn->flags |= CO_FL_SEND_PROXY;
 	}