byte-based C locale, phase 3: make MB_CUR_MAX variable to activate code

byte-based C locale, phase 3: make MB_CUR_MAX variable to activate code

this patch activates the new byte-based C locale (high bytes treated
as abstract code unit "characters" rather than decoded as multibyte
characters) by making the value of MB_CUR_MAX depend on the active
locale. for the C locale, the LC_CTYPE category pointer is null,
yielding a value of 1. all other locales yield a value of 4.