fix iconv conversions to legacy 8bit encodings

fix iconv conversions to legacy 8bit encodings

there was missing reverse-conversion logic for the case, handled
specially in the character set tables, where a byte represents a
unicode codepoint with the same value.

this patch adds code to handle the case, and refactors the two-level
10-bit table lookup for legacy character sets into a function to avoid
repeating it yet another time as part of the fix.