Define 'xtrn_sec' in the global JS scope when an section is listed

This allows the use of JS:xtrn_sec @-code in the 'which' prompt string.
For Nelgin.

exec/xtrn_sec.js

@@ -106,6 +106,8 @@ function external_program_menu(xsec)
 {
     var i,j;
 
+	js.global.xtrn_sec = xtrn_area.sec_list[xsec].name;
+
 	while(bbs.online) {
 
 		console.aborted = false;