Offer separate actions for creating OpenPGP key pair resp. S/MIME CSR

Simplify the creation of a new OpenPGP certificate resp. an S/MIME CSR
by offering two separate actions instead of a single action which in the
first steps asks the user what kind of certificate them want to create.
The old NewCertificateCommand now always creates an S/MIME CSR.

GnuPG-bug-id: 5832