change wint_t to unsigned

aside from the obvious C++ ABI purpose for this change, it also brings
musl into alignment with the compiler's idea of the definition of
wint_t (use in -Wformat), and makes the situation less awkward on ARM,
where wchar_t is unsigned.

internal code using wint_t and WEOF was checked against this change,
and while a few cases of storing WEOF into wchar_t were found, they
all seem to operate properly with the natural conversion from unsigned
to signed.

include/stdint.h

@@ -78,8 +78,8 @@ typedef uint64_t uint_least64_t;
 #define INTMAX_MAX  INT64_MAX
 #define UINTMAX_MAX UINT64_MAX
 
-#define WINT_MIN INT32_MIN
-#define WINT_MAX INT32_MAX
+#define WINT_MIN 0
+#define WINT_MAX UINT32_MAX
 
 #if L'\0'-1 > 0
 #define WCHAR_MAX (0xffffffffu+L'\0')