Currently, if xmon has no input device (as is generally the case on
G5 powermacs), and we drop into xmon as a result of a fatal exception,
it will return 1, which die() interprets as "continue without causing
an oops".  This fixes it by making xmon() return 0 in the case where
it has no input device.
Signed-off-by: NPaul Mackerras <paulus@samba.org>