non-interactive export of secret keys stored in cleartext

[Daniel Kahn Gillmor]

A shorter (squashed) series of changesets follows that address the
ability to export cleartext secret key material when it is stored in
the clear.

These should apply the the current published git master
(8127043d549a5843ea1ba2dc6da4906fc2258d53).

overview of total changes in this series:

 agent/command.c           |   7 +-
 g10/call-agent.c          |  76 +++++++++++-----
 g10/call-agent.h          |   5 +-
 g10/export.c              | 227 +++++++++++++++++++++++++++++++++++++++++++++-
 g10/gpgv.c                |   4 +-
 g10/keyedit.c             |   4 +-
 g10/keygen.c              |   4 +-
 g10/keylist.c             |  10 +-
 g10/main.h                |   1 +
 g10/sign.c                |   2 +-
 g10/test-stubs.c          |   4 +-
 tests/openpgp/export.test |  28 +-----
 12 files changed, 305 insertions(+), 67 deletions(-)