Add a USE_DIALOG global (analogous to USE_XDIALOG); useful for inserting

text into a prompt or command syntax iff dialog(1) is in-use.

Approved by:	re (glebius)

usr.sbin/bsdconfig/share/dialog.subr

@@ -2044,6 +2044,7 @@ f_dialog_menutag2index_with_help()
 f_dialog_init()
 {
 	DIALOG_SELF_INITIALIZE=
+	USE_DIALOG=1
 
 	#
 	# Clone terminal stdout so we can redirect to it from within sub-shells
@@ -2087,7 +2088,7 @@ f_dialog_init()
 	#
 	# Process `-X' command-line option
 	#
-	[ "$USE_XDIALOG" ] && DIALOG=Xdialog
+	[ "$USE_XDIALOG" ] && DIALOG=Xdialog USE_DIALOG=
 
 	#
 	# Sanity check, or die gracefully