Make screening of keyserver result work with multi-key commands.

* g10/keyserver.c (ks_retrieval_filter_arg_s): new.
(keyserver_retrieval_filter): Use new struct and check all
descriptions.
(keyserver_spawn): Pass filter arg suing the new struct.

This is a fix for commit 52303043.

The old code did only work for a single key. It failed as soon as
several keys are specified ("gpg --refresh-keys" or "gpg --recv-key A
B C").