Joel Carnat notice that GoToSocial does not like it when we sent no
User-Agent and returns an HTTP/418. This simple diff adds User-Agent to
all HTTP/HTTPS checks.

As I see it, we use "User Agents" in other places (ftp,acme-client...).
When I look at all my (aws) load balancers and checks, I get a user
agent from all of them.

diff --git a/usr.sbin/relayd/parse.y b/usr.sbin/relayd/parse.y
index fcdfb8e92e3..44ec18cbcfc 100644
--- a/usr.sbin/relayd/parse.y
+++ b/usr.sbin/relayd/parse.y
@@ -921,7 +921,7 @@ tablecheck	: ICMP			{ table->conf.check = CHECK_ICMP; }
 				YYERROR;
 			}
 			if (asprintf(&table->sendbuf,
-			    "HEAD %s HTTP/1.%c\r\n%s\r\n",
+			    "HEAD %s HTTP/1.%c\r\nUser-Agent: relayd\r\n%s\r\n",
 			    $2, strlen($3) ? '1' : '0', $3) == -1)
 				fatal("asprintf");
 			free($2);
@@ -936,7 +936,7 @@ tablecheck	: ICMP			{ table->conf.check = CHECK_ICMP; }
 			}
 			table->conf.check = CHECK_HTTP_DIGEST;
 			if (asprintf(&table->sendbuf,
-			    "GET %s HTTP/1.%c\r\n%s\r\n",
+			    "GET %s HTTP/1.%c\r\nUser-Agent: relayd\r\n%s\r\n",
 			    $2, strlen($3) ? '1' : '0', $3) == -1)
 				fatal("asprintf");
 			free($2);
diff --git a/usr.sbin/relayd/relayd.conf.5 b/usr.sbin/relayd/relayd.conf.5
index 5ae11c5ac01..943926a448a 100644
--- a/usr.sbin/relayd/relayd.conf.5
+++ b/usr.sbin/relayd/relayd.conf.5
@@ -279,6 +279,9 @@ If
 is specified, it is used as the
 .Dq Host:
 header to query a specific hostname at the target host.
+A
+.Dq User-Agent: relayd
+header is always included in the request.
 To validate the HTTP return code, use this shell command:
 .Bd -literal -offset indent
 $ echo -n "HEAD <path> HTTP/1.0\er\en\er\en" | \e