agent: Prepend the description to a PIN prompt.

* agent/divert-scd.c (has_percent0A_suffix): New.
(getpin_cb): Prepend DESC_TEXT to the prompt.
* agent/findkey.c (modify_description): Rename to ...
(agent_modify_description): this.  MAke global.  Add kludge to remove
empty parentheses from the end.
(agent_key_from_file, agent_delete_key): Adjust for above change.
* agent/pksign.c (agent_pksign_do): Modify DESC_TEXT also when
diverting to a card.

Now that we have support for multiple tokens, it is important to show
information on which key has been requested. Without that it may
happen that the PIN for a wrong card is accidentally entered.

The texts are a bit ugly, because they talk about "passphrase" but
later about entering a PIN.

A quick hack would be to s/passphrase/PIN/ in the description but that
is complicated due to i18n. Another solution might be never to talk
about PINs in the description but always about "passphrase: and only
use "PIN" or "passphrase" on the left of the entry field.