patch 8.1.0281: parsing command modifiers is not separated

Problem:    Parsing command modifiers is not separated.
Solution:   Move command modifier parsing to a separate function.

src/ex_cmds.h

@@ -1791,6 +1791,12 @@ struct exarg
     void	*cookie;	/* argument for getline() */
 #ifdef FEAT_EVAL
     struct condstack *cstack;	/* condition stack for ":if" etc. */
+#endif
+    long	verbose_save;	 // saved value of p_verbose
+    int		save_msg_silent; // saved value of msg_silent
+    int		did_esilent;	 // how many times emsg_silent was incremented
+#ifdef HAVE_SANDBOX
+    int		did_sandbox;	// when TRUE did ++sandbox
 #endif
 };