* lang/cpp/src/global.h (enum SignatureMode): Add constant SignArchive.
* lang/cpp/src/context.cpp (sigmode2sigmode): Rename to
sigflags2sigflags
(sigflags2sigflags): ... and rename argument mode to flags and treat
it as flags. Adjust the callers.
(operator<<): Change local CHECK macro to handle flags. Add new flag
to debug stream.
* lang/cpp/src/signingresult.cpp (CreatedSignature::mode): Handle
new flags (even if it cannot occur currently).