Revert "shared/conf-parser,networkd: EXTRACT_UNQUOTE|EXTRACT_RETAIN_ESCAPE → EXTRACT_...

Revert "shared/conf-parser,networkd: EXTRACT_UNQUOTE|EXTRACT_RETAIN_ESCAPE → EXTRACT_UNQUOTE"

This reverts commit 8a07b4033e5d3c86931b3dd2ddbca41118c05c60.
The tests are kept. test-networkd-conf is adjusted to pass.

This fixes #13276. I think current rules are extremely confusing, as the
case in test-networkd-conf shows. We apply some kinds of unescaping (relating
to quoting), but not others (related to escaping of special characters).
But fixing this is hard, because people have adjusted quoting to match
our rules, and if we make the rules "better", things might break in unexpected
places.