agent: Fix agent_public_key_from_file

[Daniel Kahn Gillmor]

On Wed 2015-01-07 23:06:08 -0500, NIIBE Yutaka wrote:
> Well, I don't have concrete idea about the part two.  I don't know if
> we should support arbitrary curves with parameters (for GnuPG).

fwiw, I don't think that GnuPG should support arbitrary curves with
parameters.  Group choice is a tricky and contentious issue, and GnuPG
should provide known-reasonable groups for its users, rather than
providing them with excessive flexibility that is more likely to cause
harm and confusion than to help.

     --dkg