MXS-1220: Add /users/ resource

The /users/ resource shows all user accounts that can be used with
MaxScale. This resource is further split into two resource collections,
/users/inet and /users/unix, which contain the network users and Linux
accounts respectively.

server/core/json_api.cc

@@ -137,9 +137,19 @@ json_t* mxs_json_self_link(const char* host, const char* path, const char* id)
     json_t* links = json_object();
 
     string self = host;
-    self += "/";
+
+    if (path[0] != '/')
+    {
+        self += "/";
+    }
+
     self += path;
-    self += "/";
+
+    if (self[self.length() - 1] != '/')
+    {
+        self += "/";
+    }
+
     self += id;
     json_object_set_new(links, CN_SELF, json_string(self.c_str()));