agent: Improve --debug-pinentry diagnostics

* agent/call-pinentry.c (atfork_cb): Factor code out to ...
(atfork_core): new.

We convey certain envvars directly via the environment to Pinentry and
thus they don't show up in the Assuan logging. Because we better
don't call a logging function in an atfork handle, this patch splits
the code up and uses the same code to display what was done in at fork
after the connection has been established.

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