assistant: When generating a gpg secret key, avoid hardcoding the key algorithm and...

assistant: When generating a gpg secret key, avoid hardcoding the key algorithm and size

This aims to future-proof gpg key generation. OpenPGP is in flux with a
conflict over standards ongoing. It seems not unlikely that different
systems will have different gpg commands that support different algorithms.

This also simplifies the code by using the --quick-gen-key interface rather
than the experimental batch interface. It seems less likely that
--quick-gen-key will break than an experimental interface (whose
documentation I can no longer find).

--quick-gen-key is supported since gpg 2.1.0 (2014).

Sponsored-by: Graham Spencer on Patreon