Password length and alphabet is unchanged, but the restriction to
include at least one numeric and one capital char has been removed.
This restriction is not needed by client applications,
adds code complexity, and even (insignificantly) reduces entropy.
Reason for switching to pwgen:
apg uses /dev/random instead of /dev/urandom which brings no security
benefits but can stall the generate-secrets script on low-entropy
devices due to blocking.
Since `security.rngd` has been disabled in NixOS 20.09, blocking
in generate-secrets can also appear on regular NixOS desktop systems.
Each secret file to be deployed is now backed by one local file.
This simplifies 'setup-secrets' and the secret definitions.
Also, with the old format it was not possible to add new secrets
to secrets.nix in a simple way.
Old secrets are automatically converted to the new format when running
nix-shell.
Using the new option 'nix-bitcoin.secrets', secrets are now directly
defined by the services that use them.
