refactor: replace char_u variables and functions with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/digraph.c

@@ -2087,7 +2087,7 @@ void ex_loadkeymap(exarg_T *eap)
 
   // Get each line of the sourced file, break at the end.
   for (;;) {
-    char_u *line = eap->getline(0, eap->cookie, 0, true);
+    char *line = eap->getline(0, eap->cookie, 0, true);
 
     if (line == NULL) {
       break;