dirmngr: Implement TLS over http proxies.
* dirmngr/http.c (send_request): If a http proxy is to be used, and we want to use TLS, try to use the CONNECT method to get a connection to the target server.
- GnuPG-bug-id: T2940
- Signed-off-by: Justus Winter <justus@g10code.com>