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/modinfo.h

@@ -79,6 +79,7 @@ enum mxs_module_param_type
     MXS_MODULE_PARAM_PATH, /**< Path to a file or a directory */
     MXS_MODULE_PARAM_SERVICE, /**< Service name */
     MXS_MODULE_PARAM_SERVER, /**< Server name */
+    MXS_MODULE_PARAM_SERVERLIST /**< List of server names, separated by ',' */
 };
 
 /** Maximum and minimum values for integer types */