[3.12] gh-121650: Encode newlines in headers, and verify headers are sound (GH-122233) (#122599)

* gh-121650: Encode newlines in headers, and verify headers are sound (GH-122233)

- Encode header parts that contain newlines

Per RFC 2047:

> [...] these encoding schemes allow the
> encoding of arbitrary octet values, mail readers that implement this
> decoding should also ensure that display of the decoded data on the
> recipient's terminal will not cause unwanted side-effects

It seems that the "quoted-word" scheme is a valid way to include
a newline character in a header value, just like we already allow
undecodable bytes or control characters.
They do need to be properly quoted when serialized to text, though.

- Verify that email headers are well-formed

This should fail for custom fold() implementations that aren't careful
about newlines.

Co-authored-by: Bas Bloemsaat <bas@bloemsaat.org>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
(cherry picked from commit 097633981879b3c9de9a1dd120d3aa585ecc2384)

* Document changes as made in 3.12.5