Patch by @Baughn, who noticed these imports being very slow when run
serially with many datasets, so much that the service would time out and
fail, this fixes it.
When the znapzend module was enabled for the first time with pure =
true; then the list of previous entries is empty, but xargs still tried
to execute a znapzendzetup delete command with no arguments, which made
it fail
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.
