refactor: replace char_u with char

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

src/nvim/eval.c

@@ -6273,7 +6273,7 @@ int list2fpos(typval_T *arg, pos_T *posp, int *fnump, colnr_T *curswantp, bool c
 int get_env_len(const char **arg)
 {
   const char *p;
-  for (p = *arg; vim_isIDc(*p); p++) {}
+  for (p = *arg; vim_isIDc((uint8_t)(*p)); p++) {}
   if (p == *arg) {  // No name found.
     return 0;
   }