LWIP_HTTPD_SSI macro

Set this to 1 to support SSI (Server-Side-Includes) In contrast to other http servers, this only calls a preregistered callback function (@see http_set_ssi_handler) for each tag (in the format of ) encountered in SSI-enabled pages. SSI-enabled pages must have one of the predefined SSI-enabled file extensions. All files with one of these extensions are parsed when sent. A downside of the current SSI implementation is that persistent connections don't work, as the file length is not known in advance (and httpd currently relies on the Content-Length header for persistent connections). To save memory, the maximum tag length is limited (@see LWIP_HTTPD_MAX_TAG_NAME_LEN). To save memory, the maximum insertion string length is limited (@see LWIP_HTTPD_MAX_TAG_INSERT_LEN). If this is not enought, LWIP_HTTPD_SSI_MULTIPART can be used.