curl_easy_setopt.3: slight clarification of SEEKFUNCTION

uploaded bytes with the normal read function/callback). It is also called to

rewind a stream when doing a HTTP PUT or POST with a multi-pass authentication

method. The function shall work like "fseek" or "lseek" and accepted SEEK_SET,

SEEK_CUR and SEEK_END as argument for origin, although (in 7.18.0) libcurl

only passes SEEK_SET. The callback must return 0 (CURL_SEEKFUNC_OK) on

success, 1 (CURL_SEEKFUNC_FAIL) to cause the upload operation to fail or 2

SEEK_CUR and SEEK_END as argument for origin, although libcurl currently only

passes SEEK_SET. The callback must return 0 (CURL_SEEKFUNC_OK) on success, 1

(CURL_SEEKFUNC_FAIL) to cause the upload operation to fail or 2

(CURL_SEEKFUNC_CANTSEEK) to indicate that while the seek failed, libcurl is

free to work around the problem if possible. The latter can sometimes be done

by instead reading from the input or similar.