Fetch pubkeys using hkp:// transport

In order to have a MANDATORY signature checking policy for *.rpm
packages, pubkeys (and signatures) MUST be accessible.

Using hkp:// retrieval from keys.rpm5.org is one way to achieve key distribution.

In order to automate retrieval however, the following goals must be achieved:

1) the hkp:// fetch MUST permit disabling.

2) retrieved pubkeys MUST be validated (at least at self-signing level).
    This also means that expired/revoked pubkeys MUST be ignored.

3) the hkp:// fetch MUST be efficient. Repeated lookups of failed keys need
to be avoided.