Properly sanitize --menu results (guards against Gtk library warnings from

Properly sanitize --menu results (guards against Gtk library warnings from
X11 side of things from bleeding into Xdialog(1) stderr output). It should
be duely noted that such errors are not a by-product of anything in the
Xdialog(1) utility or API, but optional libraries that it can link against
(such as Gtk1 versus Gtk2; if you compile xdialog from ports against Gtk2
AND misconfigure your fonts or generally make Gtk2 unhappy, these warning
messages can bleed into the captured stderr -- that is we we sanitize!).