<wchar.h>: Implement correct limits for different wchar_t flavors.

Try best to select correct limits while always using handpicked standard
C wchar_t bounds that match internal rune_t and standard

<wchar.h>: Implement correct limits for different wchar_t flavors.

Try best to select correct limits while always using handpicked standard
C wchar_t bounds that match internal rune_t and standard wint_t types.
It might be worth to switch to using compiler provided limits for all.
The <stdint.h> header should provide limits for all types in standard
headers, so provide wint_t (<wchar.h>, <wctype.h>) limits too.
The <machine/wchar_limits.h> idea taken from NetBSD.

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