Add helper function for reading values by field name

The helper function provides map-like access to row values. This is used
to retrieve the values for all MariaDB 10.0+ versions as there are
differences in the returned results between 10.1 and 10.2.

include/maxscale/mysql_utils.h

@@ -66,6 +66,17 @@ int mxs_mysql_query(MYSQL* conn, const char* query);
  */
 bool mxs_mysql_trim_quotes(char *s);
 
+/**
+ * Helper function for getting values by field name
+ *
+ * @param result Resultset
+ * @param row    Row where the value is read
+ * @param key    Name of the field
+ *
+ * @return The value of the field or NULL if value is not found. NULL values
+ *         are also returned as NULL pointers.
+ */
+const char* mxs_mysql_get_value(MYSQL_RES* result, MYSQL_ROW row, const char* key);
 
 typedef enum mxs_pcre_quote_approach
 {