Fix bug #416 in a different way. Our handlers code in yaz_marc8_?_conv

(generated by charconv.tcl) now returns 0 and no_read=1 when a sequence
does not match the input. The SECOND HALFs in codetables.xml produces a
non-existant entry in the conversion trie.. Hence when met, the input
byte is skipped as it should (in yaz_iconv).