bsdinstall: Fix command line argument list parsing

bsddialog(1) uses getopt_long(3) to parse command line argument list.
Add '--' to avoid errors caused by arguments (menu items) begin
with '-'.
The change is compatible with dialog(1) and Xdialog(1).

usr.sbin/bsdinstall/scripts/keymap

@@ -195,7 +195,7 @@ while :; do
 		--default-item \"\$defaultitem\"   \
 		--menu \"\$prompt\"                \
 		$height $width $rows               \
-		$menu_list                         \
+		${USE_DIALOG:+--} $menu_list       \
 		2>&1 >&$DIALOG_TERMINAL_PASSTHRU_FD
 	) || {
 		f_quietly rm -f "$KEYMAPFILE"

usr.sbin/bsdinstall/scripts/zfsboot

@@ -396,7 +396,7 @@ dialog_menu_main()
 		--default-item \"\$defaultitem\" \
 		--menu \"\$prompt\"              \
 		$height $width $rows             \
-		$menu_list                       \
+		${USE_DIALOG:+--} $menu_list     \
 		2>&1 >&$DIALOG_TERMINAL_PASSTHRU_FD
 	)
 	local retval=$?