MINOR: check: add agent-send server parameter

Causes HAProxy to emit a static string to the agent on every check,
so that you can independently control multiple services running
behind a single agent port.
diff --git a/src/server.c b/src/server.c
index c92623d..224d536 100644
--- a/src/server.c
+++ b/src/server.c
@@ -984,6 +984,9 @@
 			newsrv->check.downinter	= curproxy->defsrv.check.downinter;
 			newsrv->agent.use_ssl	= curproxy->defsrv.agent.use_ssl;
 			newsrv->agent.port	= curproxy->defsrv.agent.port;
+			if (curproxy->defsrv.agent.send_string != NULL)
+				newsrv->agent.send_string = strdup(curproxy->defsrv.agent.send_string);
+			newsrv->agent.send_string_len = curproxy->defsrv.agent.send_string_len;
 			newsrv->agent.inter	= curproxy->defsrv.agent.inter;
 			newsrv->agent.fastinter	= curproxy->defsrv.agent.fastinter;
 			newsrv->agent.downinter	= curproxy->defsrv.agent.downinter;
@@ -1052,6 +1055,14 @@
 				newsrv->agent.port = atol(args[cur_arg + 1]);
 				cur_arg += 2;
 			}
+			else if (!strcmp(args[cur_arg], "agent-send")) {
+				global.maxsock++;
+				free(newsrv->agent.send_string);
+				newsrv->agent.send_string_len = strlen(args[cur_arg + 1]);
+				newsrv->agent.send_string = calloc(1, newsrv->agent.send_string_len + 1);
+				memcpy(newsrv->agent.send_string, args[cur_arg + 1], newsrv->agent.send_string_len);
+				cur_arg += 2;
+			}
 			else if (!defsrv && !strcmp(args[cur_arg], "cookie")) {
 				newsrv->cookie = strdup(args[cur_arg + 1]);
 				newsrv->cklen = strlen(args[cur_arg + 1]);