Allow the use of the default-new-key-algo format for --quick-gen-key.
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Description

If an algo is given for --quick-gen-key only the primary key is created. We could extend extend the ALGO specification to allow for example "ed25519/cert,sign,v5+cv25519/v5" for that parameter and create a subkey with the specified algorithm (here using the v5 key format).

A further enhancement would be to allow the creation of more than one subkey. However, this requires major refactoring of keygen.c.