agent: If a Label is make sure that label is part of the prompt.

* agent/findkey.c (has_comment_expando): New.
(agent_key_from_file): Modify DESC_TEXT.

A Label entry in the keyfile is always set manually and thus we can
assume that the user wants to have this label in the prompt. In case
the prompt template does not demand a comment this patch appends a
comment to thhe template. This is a common case for on-disk keys used
by gpg.

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