dev->u.lastSlave was not signal safe since it was accessed by the DIX and
during signal handling.
Replaced with:
'dev->last.slave' for the signal handler's lastSlave (used to generate
DeviceChangedEvents), .
'dev->u.lastSlave' for the DIX lastSlave (currently only used in
change_modmap)
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>