agent: Add option --debug-pinentry.

* agent/gpg-agent.c (oDebugPinentry): New.
(opts): Add --debug-pinentry.
(parse_rereadable_options): Set that option.
* agent/call-pinentry.c (start_pinentry): Pass option to
assuan_set_flag.

This option is quite useful to see the IPC between gpg-agent and
Pinentry. Note that "debug 1024" is also required.

• Signed-off-by: Werner Koch <wk@gnupg.org>