gpg: New commands --add-recipients and --change-recipients.

* g10/gpg.c (aAddRecipients, aChangeRecipients): New consts.
(opts): Add --add-recipients and --change-recipients.
(main): Handle them.
* g10/gpg.h (struct server_control_s): Add fields modify_recipients,
clear_recipients, and last_read_ctb.
* g10/armor.c (was_armored): New.
* g10/decrypt.c (decrypt_message): Add optional arg 'remusr'.  Handle
re-encryption if desired.
* g10/encrypt.c (write_pubkey_enc): Factor info printing out to ...
(show_encrypted_for_user_info): new.
(reencrypt_to_new_recipients): New.
* g10/packet.h (struct parse_packet_ctx_s): Add fields only_fookey_enc
and last_ctb.
(init_parse_packet): Clear them.
* g10/parse-packet.c (parse): Store CTB in the context.  Early return
on pubkey_enc and symkey_enc packets if requested.
* g10/mainproc.c (proc_encrypted): Allow for PKT being NULL.  Return
early in modify-recipients mode.
(proc_encryption_packets): Add two optional args 'r_dek' and 'r_list'.
Adjust callers.  Call do_proc_packets in modify-recipients mode
depending on the optional args.
(do_proc_packets): Add arg 'keep_dek_and_list'.  Adjust callers.  Save
the last read CTB in CTRL and return after the last fooenc_enc
packets.
--

This basically works but does not yet handle symmetric encrypted
packets (symkey_enc).

GnuPG-bug-id: 1825
(Yes, this is an at least 9 year old feature request)

g10/gpg.h

@@ -122,6 +122,15 @@ struct server_control_s
   unsigned char *secret_keygrips;
   size_t secret_keygrips_len;
   int no_more_secret_keygrips;
+
+  /* This first flag is set to true if we are running a
+   * --add-recipients or --change-recipients command.  The second if
+   * it is --change-recipients.  */
+  unsigned int modify_recipients:1;
+  unsigned int clear_recipients:1;
+
+  /* Temporary used to pass the last read byte up the call chain.  */
+  int last_read_ctb;
 };