[tex4ht] xetex id update for tex4ht binary

[Karl Berry]

    I've just  tried to run the attached executable on Fefora 23 and it
    ended with ELF format error.

I expect that Fedora 23 is new enough that it inhabits a different
world, one that is intentionally binary-incompatible with everything
before it.  (I've seen this with many other "new" systems.)  I think
this behavior is reprehensible, but no one asked me :).

Alternatively, perhaps there is some compatibility library that can be
installed to make (have made) it work, but I can't guess what it is.

    \registerchar{character code}
    \registerblock{start char code}{end char code}

Wow.  I will take a look as soon as I have a chance.  

Thanks,
Karl