CP5022{0,1,2}: convert Unicode codepoints in 'user' area (0xE000-E757) correctly

Unicode has a range of 'private' codepoints which individual applications can
use for their own purposes. When they were inventing CP932, MicroSoft mapped
these 'private' or 'user' codepoints to ten new rows added to the JIS X 0208
character table. (JIS X 0208 is based on a 94x94 table; MS used rows 95-114
for private characters.)

`mbfl_filt_conv_wchar_jis_ms` converted these private codepoints to rows 85-94
rather than 95-114. The code included a link to a document on the OpenGroup
web site, dating back to 1996 [1], which proposed mapping private codepoints to
these rows. However, that is not consistent with what mbstring does when
converting CP5022x to Unicode.

There seems to be a dearth of information on CP5022x on the web. However, I
did find one (Japanese-language) page on CP50221, which states that it maps
kuten codes 0x7F21-0x927E to the 'private' Unicode codepoints [2].

As a side note, using rows higher than 95 does seem to defeat one purpose of
using an ISO-2022-JP variant: ISO-2022-JP was specifically designed to be
"7-bit clean", but once you go beyond row 95, the ku codes are 0x80 and up,
so 8 bits are needed.

[1] https://web.archive.org/web/20000229180004/http://www.opengroup.or.jp/jvc/cde/ucs-conv.html
[2] https://www.wdic.org/w/WDIC/Microsoft%20Windows%20Codepage%20%3A%2050221

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