This enables znapzend users to specify its full configuration through
NixOS options, without ever needing to use the stateful `znapzendzetup`
command.
This works by running znapzendzetup with the specified config in
ExecPre, just before the znapzend daemon is started.
There is also the `pure` option which will clear all previous znapzend setups,
making it as stateless as can get, as only the setup declared in
configuration.nix will be persisted.
