gpg: Improve decryption diagnostic for an ADSK key.

* g10/keydb.h (GET_PUBKEYBLOCK_FLAG_ADSK): New constant.
* g10/packet.h (PUBKEY_USAGE_XENC_MASK): New constant.
* g10/pubkey-enc.c (get_session_key): Consider an ADSK also as "marked
for encryption use".
(get_it): Print a note if an ADSK key was used.  Use the new
get_pubkeyblock flag.
* g10/getkey.c (struct getkey_ctx_s): Add field allow_adsk.
(get_pubkeyblock): Factor all code out to ...
(get_pubkeyblock_ext): new.
(finish_lookup): Add new arg allow_adsk and make use of it.
--

This patch solves two purposes:
- We write a note that the ADSK key was used for decryption
- We avoid running into a
  "oops: public key not found for preference check\n"
  due to ADSK keys.  The error is mostly harmless but lets gpg return
  with an exit code of 2.

g10/packet.h

@@ -60,10 +60,14 @@
 #define PUBKEY_USAGE_RENC    1024                /* Restricted encryption.  */
 #define PUBKEY_USAGE_TIME    2048                /* Timestamp use.  */
 
+
 /* The usage bits which can be derived from the algo.  */
 #define PUBKEY_USAGE_BASIC_MASK  (PUBKEY_USAGE_SIG|PUBKEY_USAGE_ENC\
                                   |PUBKEY_USAGE_CERT|PUBKEY_USAGE_AUTH)
 
+/* The usage bits which define encryption.  */
+#define PUBKEY_USAGE_XENC_MASK  (PUBKEY_USAGE_ENC | PUBKEY_USAGE_RENC)
+
 /* Bitflags to convey hints on what kind of signature is created.  */
 #define SIGNHINT_KEYSIG  1
 #define SIGNHINT_SELFSIG 2