The old code was based on QString::split and e.g. didn't handle
escapes in quoted-strings.
Write a modern recursive-descent parser to parse out type/subtype and
charset parameter. Since we don't, yet, support CFWS (see below),
recursion depth is strictly limited, so we're not susceptible to
recursion bombs here.
It currently handles only RFC9110-style grammar, but can be easily
extended to support CFWS (RFC2822-style comments and folding
white-space) or RFC2231-style continuuations, if needed.
It's a bit more general than strictly required, because I expect this
to be reused elsewhere ere long.
Pick-to: 6.7
Fixes: QTBUG-120307
Change-Id: I309928dc350a043672dffb4a259b457764c031be
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
98b034e53a