* lang/cpp/src/context.h (enum Context::ExportMode): Add value
ExportSecretSubkey.
(class Context): Add member functions exportSecretSubkeys and
startSecretSubkeyExport.
* lang/cpp/src/context.cpp (Context::exportPublicKeys,
Context::startPublicKeyExport): Return error if ExportSecretSubkey
mode flag is set.
(Context::exportSecretSubkeys, Context::startSecretSubkeyExport):
Implement.