MINOR: server: support keyword proto in 'add server' cli

Allow to specify the mux proto for a dynamic server. It must be
compatible with the backend mode to be accepted. The reg-tests has been
extended for this error case.
diff --git a/doc/management.txt b/doc/management.txt
index 6301ffa..261c74d 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1447,6 +1447,7 @@
   - pool-low-conn
   - pool-max-conn
   - pool-purge-delay
+  - proto
   - proxy-v2-options
   - send-proxy
   - send-proxy-v2