ioutil package was deprecated in Go 1.16, replace all uses with their
respective replacements.
This patch was generated with a combination of `gofmt -r`, `eg`, and
manually (for `ioutil.ReadDir`).
This patch renames courier.Procmail to courier.MDA, to make it more
obvious that the functionality is not tied to that particular MDA.
It's just for readability, there are no functional changes.
The smtpsrv fuzzer doesn't handle DATA commands particularly well:
it will continue to read but will skip lines that have STARTTLS as
content, and only really care for the first line due to a bug.
This patch fixes the handling, and moves the logic to a separate
function for readability.
