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

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

It's hard to even say what exactly this combination means. Escaping is
necessary when quoting to have quotes within the string. So the escaping of
quote characters is inherently tied to quoting. When unquoting, it seems
natural to remove escaping which was done for the quoting purposes. But with
both flags we would be expected to re-add this escaping after unqouting? Or
maybe keep the escaping which is not necessary for quoting but otherwise
present? This all seems too complicated, let's just forbid such usage and
always fully unescape when unquoting.