Allow stale master status to be assigned via maxadmin

If a master once had slaves and is in the stale status, it will not retain
this status after a restart. Without storing on-disk information, the
stale master status cannot be deduced by looking at the master
alone. Because of this, the user should be able to manually enable the
stale master status.

server/core/server.c

@@ -990,6 +990,7 @@ static struct
     { "ndb",         SERVER_NDB },
     { "maintenance", SERVER_MAINT },
     { "maint",       SERVER_MAINT },
+    { "stale",       SERVER_STALE_STATUS },
     { NULL,          0 }
 };