BUG/MINOR: mailers: negotiate SMTP, not ESMTP

As per issue #1552 the mailer code currently breaks on ESMTP multiline
responses. Let's negotiate SMTP instead.

Should be backported to 2.0.

(cherry picked from commit 1a16e4ebcb6d5848dd867a4ef3edda3760c60124)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
(cherry picked from commit 667c78079f0a6e33a670495c8f0728400c98700c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/mailers.c b/src/mailers.c
index 3df02f0..4138bae 100644
--- a/src/mailers.c
+++ b/src/mailers.c
@@ -195,7 +195,7 @@
 		goto error;
 
 	{
-		const char * const strs[4] = { "EHLO ", p->email_alert.myhostname, "\r\n" };
+		const char * const strs[4] = { "HELO ", p->email_alert.myhostname, "\r\n" };
 		if (!add_tcpcheck_send_strs(&alert->rules, strs))
 			goto error;
 	}