Build the sub's internal code (from group prefix and sub suffix) when saving

... QWKnet hub sub-boards

The internal code is not "prepped" when read by SCFG, so the code is actually
blank (empty string) when saved in SCFG and is thus not useable for writing
the value to .ini, as in this case.

This fixes issue #441 (for the new_config_format branch) as reported by Nelgin

src/sbbs3/scfgsave.c

@@ -489,7 +489,11 @@ BOOL write_msgs_cfg(scfg_t* cfg, int backup_level)
 				continue;
 			SAFEPRINTF2(name, "qhubsub:%s:%u", cfg->qhub[i]->id, cfg->qhub[i]->conf[j]);
 			str_list_t section = strListInit();
-			iniSetString(&section, name, "sub", cfg->sub[subnum]->code, NULL);
+			char code[LEN_EXTCODE + 1];
+			SAFEPRINTF2(code,"%s%s"
+				,cfg->grp[cfg->sub[subnum]->grp]->code_prefix
+				,cfg->sub[subnum]->code_suffix);
+			iniSetString(&section, name, "sub", code, NULL);
 			iniSetHexInt(&section, name, "settings", cfg->qhub[i]->mode[j], NULL);
 			strListMerge(&ini, section);
 			free(section);