gpg-agent: Add restricted connection feature.

* agent/agent.h (opt): Add field extra_socket.
(server_control_s): Add field restricted.
* agent/command.c: Check restricted flag on many commands.
* agent/gpg-agent.c (oExtraSocket): New.
(opts): Add option --extra-socket.
(socket_name_extra): New.
(cleanup): Cleanup that socket name.
(main): Implement oExtraSocket.
(create_socket_name): Add arg homedir and change all callers.
(create_server_socket): Rename arg is_ssh to primary and change
callers.
(start_connection_thread): Take ctrl as arg.
(start_connection_thread_std): New.
(start_connection_thread_extra): New.
(handle_connections): Add arg listen_fd_extra and replace the
connection starting code by parameterized loop.
* common/asshelp.c (start_new_gpg_agent): Detect the use of the
restricted mode and don't fail on sending the pinentry environment.

* common/util.h (GPG_ERR_FORBIDDEN): New.

common/util.h

@@ -35,6 +35,12 @@
 #include <errno.h>  /* We need errno.  */
 #include <gpg-error.h> /* We need gpg_error_t and estream. */
 
+/* These error codes are used but not defined in the required
+   libgpg-error version.  Define them here. */
+#if GPG_ERROR_VERSION_NUMBER < 0x011200  /* 1.18 */
+# define GPG_ERR_FORBIDDEN    251
+#endif
+
 
 /* Hash function used with libksba. */
 #define HASH_FNC ((void (*)(void *, const void*,size_t))gcry_md_write)