This is a staging area for further refinement of pinentry.el, until it
gets included in Emacs proper or ELPA.

See the discussion on the GnuPG mailing list:
https://lists.gnupg.org/pipermail/gnupg-devel/2015-May/029875.html
https://lists.gnupg.org/pipermail/gnupg-devel/2015-June/029915.html