Refactor some lower level functions in tde_keyring

Previously write_key_provider_info() was a bit of a "do everything"
function that had very different behavior depending on what parameters
was passed to it. This commit reworks it to a "dumb" function that just
writes the data without asking questions and have the callers take
responsibility for data validity.

This is to make it easier to validate the data in different ways
depending on the caller's needs without further complicating
write_key_provider_info().

Author: Anders Åstrand