feat(ui): allow to set the highlight namespace per window

- reimplement 'winhl' in terms of highlight namespaces
- check for EOF in screen tests (to indicate a likely crash)

src/nvim/vim.h

@@ -275,8 +275,8 @@ enum { FOLD_TEXT_LEN = 51, };  //!< buffer size for get_foldtext()
                                           (const char *)(y), \
                                           (size_t)(n))
 
-// Enums need a typecast to be used as array index (for Ultrix).
-#define HL_ATTR(n)      highlight_attr[(int)(n)]
+// Enums need a typecast to be used as array index.
+#define HL_ATTR(n)      hl_attr_active[(int)(n)]
 
 /// Maximum number of bytes in a multi-byte character.  It can be one 32-bit
 /// character of up to 6 bytes, or one 16-bit character of up to three bytes