I made some mistakes on this code, which meant that not everything which
should be tested was actually being tested.

There is no meaningful difference between these and UCS-{2,4}. They are
just a little bit more lax about passing errors silently. They also have
no known use.

Alias to UCS-{2,4} in case someone, somewhere is using them.

- Reject otherwise valid kuten codes which don't map to anything in JIS X 0208.
- Handle truncated multi-byte characters as an error.
- Convert Shift-JIS 0x7E to Unicode 0x203E (overline) as recommended by the
  Unicode Consortium, and as iconv does.
- Convert Shift-JIS 0x5C to Unicode 0xA5 (yen sign) as recommended by the
  Unicode Consortium, and as iconv does.
  (NOTE: This will affect PHP scripts which use an internal encoding of
  Shift-JIS! PHP assigns a special meaning to 0x5C, the backslash. For example,
  it is used for escapes in double-quoted strings. Mapping the Shift-JIS yen
  sign to the Unicode yen sign means the yen sign will not be usable for
  C escapes in double-quoted strings. Japanese PHP programmers who want to
  write their source code in Shift-JIS for some strange reason will have to
  use the JIS X 0208 backlash or 'REVERSE SOLIDUS' character for their C
  escapes.)
- Convert Unicode 0x5C (backslash) to Shift-JIS 0x815F (reverse solidus).
- Immediately handle error if first Shift-JIS byte is over 0xEF, rather than
  waiting to see the next byte. (Previously, the value used was 0xFC, which is
  the limit for the 2nd byte and not the 1st byte of a multi-byte character.)
- Don't allow 'control characters' to appear in the middle of a multi-byte
  character.

The test case for bug 47399 is now obsolete. That test assumed that a number
of Shift-JIS byte sequences which don't map to any character were 'valid'
(because the byte values were within the legal ranges).

- Don't allow control characters to appear in the middle of a multi-byte
  character. (A strange feature, or perhaps misfeature, of mbstring which is
  not present in other libraries such as iconv.)
- When checking whether string is valid, reject kuten codes which do not
  map to any character, whether converting from EUC-JP to another encoding,
  or converting another encoding which uses JIS X 0208/0212 charsets to
  EUC-JP.
- Truncated multi-byte characters are treated as an error.

mbstring had an 'identify filter' for almost every supported text encoding
which was used when auto-detecting the most likely encoding for a string.
It would run over the string and set a 'flag' if it saw anything which
did not appear likely to be the encoding in question.

One problem with this scheme was that encodings which merely appeared
less likely to be the correct one were completely rejected, even if there
was no better candidate. Another problem was that the 'identify filters'
had a huge amount of code duplication with the 'conversion filters'.

Eliminate the identify filters. Instead, when auto-detecting text
encoding, use conversion filters to see whether the input string is valid
in candidate encodings or not. At the same type, watch the type of
codepoints which the string decodes to and mark it as less likely if
non-printable characters (ESC, form feed, bell, etc.) or 'private use
area' codepoints are seen.

Interestingly, one old test case in which JIS text was misidentified
as UTF-8 (and this wrong behavior was enshrined in the test) was 'fixed'
and the JIS string is now auto-detected as JIS.