import: optionally pull .verity + .roothash.p7s data when downloading

We already had support for downlading a .nspawn and a .roothash file,
let's make the set complete, and also download .verity + roothash.p7s if
it exists, as nspawn consumes that.

Since there are now four kinds of additional resources to acquire, let's
introduce a PullFlags flags value for this instead of separate 'bool'
variables, it's just too many to always pass those around on the
function parameter list.