Fix mbstring support for ISO-2022-JP-KDDI encoding

- Treat it as an error if a multi-byte character or escape sequence is truncated
- When converting other encodings to ISO-2022-JP-KDDI, don't swallow trailing
  hash characters or digits
- Don't allow 'control' characters to appear in the middle of a multi-byte char

Note: I was not able to find any kind of official or even semi-official
specification for this legacy encoding. Therefore, the test suite for
ISO-2022-JP-KDDI is based largely on the behavior of the existing code.

Verifying the correctness of program code in this way is very questionable.
In a sense, all you are proving is that the code "does what it does". However,
the test suite will still expose any unintended _changes_ to behavior.

Author: alexdowad