gpg-agent: SSH certificate support

[Werner Koch]

On Fri,  5 Aug 2016 10:48, gniibe at fsij.org said:
>   read_key_file (const unsigned char *grip, gcry_sexp_t *result, int *ssh)
>
> When SSH is not NULL, it means allowing returning SSH certificate.

I would suggest to change to "char **ssh" and return a malloced buffer
with the certificate (in some encoding).  The creation and parsing of
the s-expressions is quite complicate when not using Lisp and we need to
return that data anyway as a plain buffer.  This way we reduce the risk
of introducing bugs in code paths not related to the ssh certificates.

To be future proof an strlist_t could also be used which would allow to
return several certifciates or other info.

> Anyway, here is a clean up part.

Thanks.


Shalom-Salam,

   Werner

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