[MINOR] acl: add srv_conn acl to count connections on a
specific backend server

These ACLs are used to check the number of active connections on the specified server in the specified backend.
diff --git a/src/backend.c b/src/backend.c
index 4b923a7..d76324f 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1551,6 +1551,17 @@
 	return 1;
 }
 
+/* set test->i to the number of concurrent connections on the server in the backend */
+static int
+acl_fetch_srv_conn(struct proxy *px, struct session *l4, void *l7, int dir,
+		  struct acl_expr *expr, struct acl_test *test)
+{
+	struct server *srv = expr->arg.srv;
+
+	test->i = srv->cur_sess;
+	return 1;
+}
+
 /* Note: must not be declared <const> as its list will be overwritten */
 static struct acl_kw_list acl_kws = {{ },{
 	{ "nbsrv",        acl_parse_int,     acl_fetch_nbsrv,          acl_match_int,     ACL_USE_NOTHING },
@@ -1562,6 +1573,7 @@
 	{ "avg_queue",    acl_parse_int,     acl_fetch_avg_queue_size, acl_match_int,     ACL_USE_NOTHING },
 	{ "srv_is_up",    acl_parse_nothing, acl_fetch_srv_is_up,      acl_match_nothing, ACL_USE_NOTHING },
 	{ "srv_id",       acl_parse_int,     acl_fetch_srv_id,         acl_match_int,     ACL_USE_RTR_INTERNAL },
+	{ "srv_conn",     acl_parse_int,     acl_fetch_srv_conn,       acl_match_int,     ACL_USE_NOTHING },
 	{ NULL, NULL, NULL, NULL },
 }};