Core: support several "error_log" directives. When several "error_log" directives are specified in the same configuration block, logs are written to all files with a matching log level. All logs are stored in the singly-linked list that is sorted by log level in the descending order. Specific debug levels (NGX_LOG_DEBUG_HTTP,EVENT, etc.) are not supported if several "error_log" directives are specified. In this case all logs will use debug level that has largest absolute value.