dirmngr: fix handling of HTTPS redirections during HKP

* dirmngr/ks-engine-hkp.c (send_request): Reinitialize HTTP session when
following a HTTP redirection.

inspired by patch from Damien Goutte-Gattat <dgouttegattat@incenp.org>

GnuPG-Bug_id: 4566

Originally applied to 2.2. Here a minor conflict fix was needed.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>