[COMMIT] Support new IGNORE-INVALID-SEQUENCESP argument, #'query-coding-region.

My XEmacs crashed without a core file when sending this. (No, I wasn’t running a version with any of the code I include.) I *believe* VM has a bug where forwarding a message as message/rfc822 breaks MIME character encoding in the forwarded message, especially if the enclosing message is entirely ASCII; I’m sending the enclosing message as UTF-8 to try and avoid this, but if the non-ASCII characters are still corrupt, my apologies.