* can wrap around to the start, and the xbuf may have its ".size" reduced
* (presumably by 1 byte) if the iconv code doesn't have space to store a
* multi-byte character at the physical end of the ".buf" (though no reducing
* can wrap around to the start, and the xbuf may have its ".size" reduced
* (presumably by 1 byte) if the iconv code doesn't have space to store a
* multi-byte character at the physical end of the ".buf" (though no reducing
*
* If ICB_EXPAND_OUT is set in "flags", the "out" xbuf will be allocated if
* empty, and (as long as ICB_CIRCULAR_OUT is not set) expanded if too small.
*
* If ICB_EXPAND_OUT is set in "flags", the "out" xbuf will be allocated if
* empty, and (as long as ICB_CIRCULAR_OUT is not set) expanded if too small.