Fix conversion of EUC-CN text (and add test suite)

- Flag truncated multi-byte characters as erroneous.
- Don't allow ASCII control characters to appear in the middle of a
multi-by

Fix conversion of EUC-CN text (and add test suite)

- Flag truncated multi-byte characters as erroneous.
- Don't allow ASCII control characters to appear in the middle of a
multi-byte character.
- There was a bug whereby some unrecognized Unicode codepoints would be
passed through unchanged to the output when converting Unicode to
EUC-CN.
- Stick to the original EUC-CN standard, rather than CP936 (an extended
version invented by MS).

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