The `config_read` function currently performs both reading the
on-disk config file as well as parsing the retrieved buffer
contents. To optimize how we refresh our config entries from an
in-memory buffer, we need to be able to directly parse buffers,
though, without involving any on-disk files at all.

Extract a new function `config_read_buffer` that sets up the
parsing logic and then parses config entries from a buffer, only.
Have `config_read` use it to avoid duplicated logic.