agent: Make --allow-loopback-pinentry the default.

* agent/gpg-agent.c (oNoAllowLoopbackPinentry): New.
(opts): Add --no-allow-loopback-pinentry.  Hide
description of --allow-loopback-pinentry.
(parse_rereadable_options): Set opt.allow_loopback_pinentry by
default.
(main): Replace allow-loopback-pinentry by no-allow-loopback-pinentry
in the gpgconf list.
* tools/gpgconf-comp.c (gc_options_gpg_agent): Ditto.

Given that a user can anyway change that options in the gpg-agent.conf
file and that gpg needs to be invoked with --pinentry-mode=loopback
the former default does not make much sense - in that option is useful
at all. There was a discussion of this topic on gnupg-devel in April
without a clear result. So we try this new default and just in case
real problems are found for the majority of installations, we can
revert that. The new default is also aligned with GnuPG's policy to
make its use easier and only require users with very high security
standards to tweak certain options (those users have anyway modeled
their threat model and configured their software according to this).

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