gpg: Add pinentry-mode feature.

* g10/gpg.c: Include shareddefs.h.
(main): Add option --pinentry-mode.
* g10/options.h (struct opt): Add field pinentry_mode.
* g10/passphrase.c: Include shareddefs.h.
(have_static_passphrase): Take care of loopback pinentry_mode.
(read_passphrase_from_fd): Ditto.
(get_static_passphrase): New.
(passphrase_to_dek_ext): Factor some code out to ...
(emit_status_need_passphrase): new.
* g10/call-agent.c (start_agent): Send the pinentry mode.
(default_inq_cb): Take care of the PASSPHRASE inquiry.  Return a
proper error code.
(agent_pksign): Add args keyid, mainkeyid and pubkey_algo.
(agent_pkdecrypt): Ditto.
* g10/pubkey-enc.c (get_it): Pass new args.
* g10/sign.c (do_sign): Pass new args.

* g10/call-agent.c (struct default_inq_parm_s): New.  Change all
similar structs to reference this one.  Change all users and inquire
callback to use this struct, instead of NULL or some undefined but not
used structs.  This change will help to eventually get rid of global
variables.
--

This new features allows to use gpg without a Pinentry.  As a
prerequisite the agent must be configured to allow the loopback
pinentry mode (option --allow-loopback-pinentry).  For example

  gpg2 --pinentry-mode=loopback FILE.gpg

may be used to decrypt FILE.gpg while entering the passphrase on the
tty.  If batch is used, --passphrase et al. may be used, if
--command-fd is used, the passphrase may be provided by another
process.  Note that there are no try-again prompts in case of a bad
passphrase.

doc/gpg.texi

@@ -2611,6 +2611,26 @@ Note that this passphrase is only used if the option @option{--batch}
 has also been given.  This is different from @command{gpg}.
 @end ifclear
 
+@ifset gpgtwoone
+@item --pinentry-mode @code{mode}
+@opindex pinentry-mode
+Set the pinentry mode to @code{mode}.  Allowed values for @code{mode}
+are:
+@table @asis
+  @item default
+  Use the default of the agent, which is @code{ask}.
+  @item ask
+  Force the use of the Pinentry.
+  @item cancel
+  Emulate use of Pinentry's cancel button.
+  @item error
+  Return a Pinentry error (``No Pinentry'').
+  @item loopback
+  Redirect Pinentry queries to the caller.  Note that in contrast to
+  Pinentry the user is not prompted again if he enters a bad password.
+@end table
+@end ifset
+
 @item --command-fd @code{n}
 @opindex command-fd
 This is a replacement for the deprecated shared-memory IPC mode.