Add new parameter type MXS_MODULE_PARAM_SERVERLIST

This is a list of servers, separated by commas. When queried as a
config setting, returns a null-terminated array of SERVER*:s. The
commit includes a serverlist parsing function, which should probably
be used anywhere a similarly formed string is parsed.

include/maxscale/config.h

@@ -205,6 +205,17 @@ struct service* config_get_service(const MXS_CONFIG_PARAMETER *params, const cha
  */
 struct server* config_get_server(const MXS_CONFIG_PARAMETER *params, const char *key);
 
+/**
+ * @brief Get an array of servers. The caller should free the returned array, but not
+ * the array elements.
+ *
+ * @param params List of configuration parameters
+ * @param key Parameter name
+ *
+ * @return Pointer to a null-terminated array of servers, or null if none found.
+ */
+struct server** config_get_serverlist(const MXS_CONFIG_PARAMETER *params, const char *key);
+
 /**
  * @brief Get copy of parameter value if it is defined
  *