Extending internal CURL wrapper for POST upload

This commit adds POST upload capabilities to the internal libcurl wrapper, which
are necessary for uploading RFC3161 Time Stamping Queries to a Time Stamping
Authority.

Therefore, the internal struct http_get_options is renamed to
http_request_options and extended by 2 strbuf fields named `postfields` and
`post_content_type`. If either one of them is non-NULL, the corresponding
curl_easy_setopt() functions are called.

Signed-off-by: Anton Würfel <anton.wuerfel@fau.de>
Signed-off-by: Phillip Raffeck <phillip.raffeck@fau.de>