gpg: Skip keys found via ADSKs.

* g10/encrypt.c (write_pubkey_enc): Indicate encryption to an ADSK.
* g10/getkey.c (finish_lookup): Skip ADKS keys.

If a key is searched by fingerprint or keyid and it happens that this
is an ADSK (subkey with the RENC usage), we need to skip this key
because it is not the key we actually want to encrypt to. The actual
ADSK key is taken later by looking at all subkeys of the actual
selected key.

This is related to

• GnuPG-bug-id: T6504