agent: Add strings for use by future Pinentry versions.

* agent/call-pinentry.c (start_pinentry): Add more strings.

We do this so that translations of these strings will be available at
the time a pinentry implements features which require these strings.

This is a backport from master
commit 02d5e1205489aa5027a87a64552eaf15984dc22d
without the mode flag in the table.

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