No guard against changing the Z-alphabet late in compilation

[erkyrath]

This compiles:

[ Main;
	print "Hello. This is a test of Class and String.^";
];

Zcharacter "abCdefgHtjklmnopqrsiuvwxyz"
           "ABcDEFGhIJKLMNOPQRSTUVWXYZ"
           "0123456789!$&*():;.,<>`";

The output is terrible, obviously. We should have an error if you try to change the alphabet after compiling strings.

This interacts with #276 , because Inform compiles a few strings at setup time! A naive fix would break the Zcharacter directive entirely -- because the Zcharacter directive can always corrupt those few strings.

Needs some careful sequencing.

[heasm66]

I'm not entirely sure what you mean, but the Zcharacter directive could issue a warning if text translation already is started.

   @@ -1189,6 +1202,8 extern int parse_given_directive(int internal_flag)
               error("The Zcharacter directive has no meaning in Glulx.");
               panic_mode_error_recovery(); return FALSE;
           }
+          if (total_chars_trans > 0)
+             warning("Zcharacter change alphabet after text translation already begun.");

           directive_keywords.enabled = TRUE;
           get_next_token();

Of course, this change after #276, otherwise we get a warning every time.