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/wchar.h

@@ -36,7 +36,7 @@ extern "C" {
 #define NULL 0L
 
 #undef WEOF
-#define WEOF (-1)
+#define WEOF 0xffffffffU
 
 typedef struct __mbstate_t
 {