- fixed copy/nocopy for SSI files;
- added LWIP_HTTPD_SSI_MULTIPART (default off): when on, SSI functions have one more argument that is used as state for an SSI tag to return more than LWIP_HTTPD_MAX_TAG_INSERT_LEN bytes (the handler function is then called multiple times);
- added LWIP_HTTPD_POST_MANUAL_WND (default off): when on, post callbacks can throttle RX speed by manually sending window updates (i.e. tcp_recved is called manually by calling httpd_post_data_recved);
- added LWIP_HTTPD_FILE_STATE (default off): when on, fs_open/fs_close call application defined functions fs_state_init/fs_state_free (fs_state_init returns a void pointer that is passed to every call to an SSI handler);
- compacted the code a bit by moving the code that calls tcp_write() in a loop to its own function;
- error files may have the extension shtml, too;
- fixed that returning default 404 page did not work any more;
