* src/engine-backend.h (engine_ops.sign): Rename argument mode to
flags.
* src/engine-gpg.c (gpg_sign): Rename argument mode to flags. Check
for invalid combination of flags.
* src/engine-gpgsm.c (gpgsm_sign): Rename argument mode to flags.
Check for unsupported flags.
* src/engine-uiserver.c (gpgsm_sign): Rename argument mode to flags.
Check for unsupported flags.
* src/engine.c, src/engine.h (_gpgme_engine_op_sign): Rename argument
mode to flags.
* src/gpgme.h.in (GPGME_SIG_MODE_ARCHIVE): Change value to 4.
(gpgme_op_sign_start, gpgme_op_sign): Rename argument mode to flags.
* src/sign.c (sign_start): Rename argument mode to flags. Adjust
check for invalid flags.
(gpgme_op_sign_start, gpgme_op_sign): Rename argument mode to flags.