Remove project int types: Case short_u: Replace with uint16_t.

- Replace short_u with uint16_t (same size, should give no problems).
- When possible, remove explicit downcasts so that they are found when
  flag -Wconversion enabled.
- Remove short_u typedef.

Requested in #459.

src/normal.c

@@ -218,7 +218,7 @@ typedef void (*nv_func_T)(cmdarg_T *cap);
 static const struct nv_cmd {
   int cmd_char;                 /* (first) command character */
   nv_func_T cmd_func;           /* function for this command */
-  short_u cmd_flags;            /* NV_ flags */
+  uint16_t cmd_flags;           /* NV_ flags */
   short cmd_arg;                /* value for ca.arg */
 } nv_cmds[] =
 {