gpg: Set a limit of 5 to the number of keys imported from the WKD.

* g10/import.c (import): Limit the number of considered keys to 5.
(import_one): Return the first fingerprint in case of WKD.

The Web Key Directory should carry only one key. However, some
providers like to put old or expired keys also into the WKD. I don't
thunk that this is a good idea but I heard claims that this is needed
for them to migrate existing key data bases.

This patch puts a limit on 5 on it (we had none right now) and also
fixes the issue that gpg could not work immediately with the requested
key because the code uses the fingerprint of the key to use the
imported key. Now the first key is used. On a second try (w/o
accessing the WKD) the regular key selection mechanism would be in
effect. I think this is the most conservative approach. Let's see
whether it helps.

• Signed-off-by: Werner Koch <wk@gnupg.org>