fetching the requested key from keyservers

[Andy Isaacson]

If the user asks for a specific fingerprint (giving the full hash to
--recv-key), it would be nice if gnupg checked the returned data from
the keyserver and only accepted keys matching the requested fingerprint.

Currently (1.4.14 and 2.0.20) whatever the keyserver returns is added to
pubring, without filtering.

https://bugs.g10code.com/gnupg/issue1579

It would be really lovely to get the patch for this issue merged; while
I'm not a gnupg developer, I did review the patch and it looks good to
me.

-andy