MINOR: mailers: use <CRLF> for all line endings

Not doing so causes issues with Exchange2013 not processing the message
body from the email. Specification seems to specify that as correct
behavior : https://www.ietf.org/rfc/rfc2821.txt # 2.3.7 Lines

 > SMTP client implementations MUST NOT transmit "bare" "CR" or "LF" characters.

This patch should be backported to 1.6.

Acked-by: Simon Horman <horms@verge.net.au>
diff --git a/src/checks.c b/src/checks.c
index 9d6d33e..2cfb01f 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -3249,12 +3249,12 @@
 		struct tm tm;
 		char datestr[48];
 		const char * const strs[18] = {
-			"From: ", p->email_alert.from, "\n",
-			"To: ", p->email_alert.to, "\n",
-			"Date: ", datestr, "\n",
-			"Subject: [HAproxy Alert] ", msg, "\n",
-			"\n",
-			msg, "\n",
+			"From: ", p->email_alert.from, "\r\n",
+			"To: ", p->email_alert.to, "\r\n",
+			"Date: ", datestr, "\r\n",
+			"Subject: [HAproxy Alert] ", msg, "\r\n",
+			"\r\n",
+			msg, "\r\n",
 			"\r\n",
 			".\r\n",
 			NULL