Add EXECDIR, DATADIR, and XTRNDIR to DOSemu command replacement tokens

As requested. This closes issue #416

src/sbbs3/xtrn.cpp

@@ -1527,6 +1527,9 @@ int sbbs_t::external(const char* cmdline, long mode, const char* startup_dir)
 			{(char*)"TERM", dosterm}, 
 			{(char*)"CTRLDIR", cfg.ctrl_dir},
 			{(char*)"NODEDIR", cfg.node_dir},
+			{(char*)"EXECDIR", cfg.exec_dir},
+			{(char*)"DATADIR", cfg.data_dir},
+			{(char*)"XTRNDIR", xtrndir},
 			{(char*)"DOSEMUBIN", dosemubinloc},
 			{(char*)"VIRTUALCONF", virtualconf},
 			{(char*)"DOSEMUCONF", dosemuconf},