gpg-agent: Use "pinentry-basic" as fallback.

* common/homedir.c (get_default_pinentry_name): New.
(gnupg_module_name): Use that for the default pinentry.
(gnupg_module_name_flush_some): New.
* agent/gpg-agent.c (agent_sighup_action): Flush some module names.
* agent/call-pinentry.c (start_pinentry): Do not modify
opt.pinentry_program.

The idea with this change is that under Windows we can install a
simple native Windows pinentry as "pinentry-basic" and a full GUI
version may then later install pinentry-gtk etc which would then
automatically be used.

Unfortunately installing another pinentry from a different package
would clobber the GnuPG core directory which is not nice. To fix that
we would need to agree on standard installation directories for GUIs
to also look there.

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