fix: reject opening directories as files in ext2 and fat32, add mbrtowc to libc

programs/include/libc/wchar.h

@@ -21,6 +21,7 @@ typedef struct {
 /* Byte-oriented C locale only: one byte, one character. */
 size_t mbstowcs(wchar_t *dst, const char *src, size_t n);
 int    mblen(const char *s, size_t n);
+size_t mbrtowc(wchar_t *pwc, const char *s, size_t n, mbstate_t *ps);
 
 #ifdef __cplusplus
 }