dirmngr: Honor http keyserver URLs.

* dirmngr/http.c (parse_uri): Keep an unmodified copy of the URI.
* dirmngr/http.h (struct parsed_uri_s): New field 'original'.
* dirmngr/ks-action.c (ks_action_get): Properly handle http and https
URLs.

If a key has a http or https URL as preferred keyserver, fetch the key
from there. Previously, dirmngr unconditionally interpreted these
URLs as hkp servers.

• GnuPG-bug-id: T2924
• Signed-off-by: Justus Winter <justus@g10code.com>