Fetch keys via HTTP, avoid dirmngr
This implements fetching keys via HTTP (without dirmngr), and only use gpg for importing them. It also features looking up keys at a VKS keyserver (e.g. keys.openpgp.org) first, before looking them up at a SKS keyserver.
It adds a new dependency: (lib)curl. That is required by the networking library, which was chosen because it allows to specify socks-proxies (with which one can route traffic via TOR).
It's a breaking change because we drop a config option (
keyserver) and also change the way how network requests can be influenced (no
dirmngr.conf anymore but new config options).