cpp: Respect decrypt flags in new functions
cpp: Respect decrypt flags in new functions

* lang/cpp/src/context.cpp: Respect directly provided flags
in the new decrypt functions.

--
Overlooked in the initial commit. Also fixed the according
unused variable warnings.

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme