*: Partial string handling refactoring

Main points:

- Replace `char_u` with `char` in some cases.
- Remove `str[len] = NUL` hack in some cases when `str` may be considered
  `const`.

src/nvim/misc1.c

@@ -2545,7 +2545,7 @@ void vim_beep(unsigned val)
      * function give the user a hint where the beep comes from. */
     if (vim_strchr(p_debug, 'e') != NULL) {
       msg_source(hl_attr(HLF_W));
-      msg_attr((char_u *)_("Beep!"), hl_attr(HLF_W));
+      msg_attr(_("Beep!"), hl_attr(HLF_W));
     }
   }
 }