[Lynx-dev] Support .mhtml

[Dan Jacobson]

On my chomebook with chrome I save a file, and it makes a "mhtml"
file. E.g., Downloads/Mapping USA 2024 _ OpenStreetMap US.mhtml .
Later offline, chrome has no problem opening that file.
But in lynx or w3m one just sees

From: <Saved by Blink>
Snapshot-Content-Location: https://openstreetmap.us/events/mapping-usa/
Subject: Mapping USA 2024 | OpenStreetMap US
Date: Thu, 11 Jan 2024 22:15:27 +0800
MIME-Version: 1.0
Content-Type: multipart/related;
        type="text/html";

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