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: 6882
Backported-from-master: d30e345692

Note that we still use the NO_AKL and not the newer TRY_LDAP in 2.2.
We may want to backport that change as well.

g10/packet.h

@@ -400,10 +400,9 @@ typedef struct
      when serializing.  (Serialized.)  */
   byte    version;
   byte    selfsigversion; /* highest version of all of the self-sigs */
-  /* The public key algorithm.  (Serialized.)  */
-  byte    pubkey_algo;
+  byte    pubkey_algo;    /* The public key algorithm.  (PGP format)  */
   u16     pubkey_usage;   /* carries the usage info.            */
-  byte    req_usage;      /* hack to pass a request to getkey() */
+  u16     req_usage;      /* hack to pass a request to getkey() */
   u32     has_expired;    /* set to the expiration date if expired */
   /* keyid of the primary key.  Never access this value directly.
      Instead, use pk_main_keyid().  */