gpg: Allow the use of an ADSK subkey as ADSK subkey.

* g10/packet.h (PKT_public_key): Increased size of req_usage to 16.
* g10/getkey.c (key_byname): Set allow_adsk in the context if ir was
requested via req_usage.
(finish_lookup): Allow RENC usage matching.
* g10/keyedit.c (append_adsk_to_key): Adjust the assert.
* g10/keygen.c (prepare_adsk): Also allow to find an RENC subkey.

If an ADSK is to be added it may happen that an ADSK subkey is found
first and this should then be used even that it does not have the E
usage. However, it used to have that E usage when it was added.

While testing this I found another pecularity: If you do

gpg -k ADSK_SUBKEY_FPR

without the '!' suffix and no corresponding encryption subkey is dound,
you will get an unusabe key error. I hesitate to fix that due to
possible side-effects.

• GnuPG-bug-id: T6882