agent: Clean up pinentry access locking.

* agent/agent.h (struct server_control_s): Rename PINENTRY_ACTIVE.
* agent/call-pinentry.c (entry_owner): Remove.
(agent_reset_query): Use thread private object of PINENTRY_ACTIVE.
(unlock_pinentry): Add CTRL to arguments to access thread private.
Check and decrement PINENTRY_ACTIVE for recursive use.
(start_pinentry): Check and increment PINENTRY_ACTIVE for recursion.
(agent_askpin): Follow the change of unlock_pinentry API.
(agent_get_passphrase, agent_get_confirmation): Likewise.
(agent_show_message, agent_popup_message_start): Likewise.
(agent_popup_message_stop, agent_clear_passphrase): Likewise.

--

We use the member PINENTRY_ACTIVE as a thread private object.
It's only valid for a single thread at a time.

It would be possible to have a thread shared object of
PINENTRY_ACTIVE, keeping ENTRY_OWNER for distinguishing its
owner (which is also a thread shared object).  But, in this case,
access to ENTRY_OWNER is tricky (only comparison to accessing thread
would be OK with no lock), or we need to introduce another lock for
accessing ENTRY_OWNER, which complicates the code too much.

So, simply have a thread private object for recursive pinentry access.

GnuPG-bug-id: 3190
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
(cherry picked from commit fb7828676c)

agent/agent.h

@@ -255,8 +255,9 @@ struct server_control_s
      count. */
   unsigned long s2k_count;
 
-  /* Recursion level of pinentry.  */
-  int pinentry_level;
+  /* If pinentry is active for this thread.  It can be more than 1,
+     when pinentry is called recursively.  */
+  int pinentry_active;
 };