security/pinentry: Users who enable the non-default GNOME3 option will now
have to install security/pinentry-gnome3 manually. This has been done to break
a dependency loop that was introduced in r553735 where security/gcr added a
build dependency on security/gnupg.

Before r553735:
security/gnupg -> security/pinentry -> security/pinentry-gnome3 -> security/gcr

After r553735:
security/gnupg -> security/pinentry -> security/pinentry-gnome3 -> security/gcr
-> security/gnupg

Now:
security/gnupg -> security/pinentry
security/pinentry-gnome3 -> security/gcr -> security/gnupg

A pkg-message has been added to remind users of the GNOME3 option to install the
required package manually and to deinstall it if no longer needed.

PR: 250945
Reported by: <sdalu@sdalu.com>