Serialize SandboxMode enum to string for JSON
Serialize SandboxMode enum to string for JSON

Rather than showing an integer as the default, instead show the boolean
referenced in the description.

The nix.conf.5 manpage used to show "default: 0", which is unnecessarily
opaque and confusing (doesn't 0 mean false, even though the default is
true?); now it properly shows that the default is true.

NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix