backport akuchling's checkin of

backport akuchling's checkin of
    revision 1.29 of asyncore.py

[Bug #517554] When a signal happens during the select call in
	asyncore.poll, the select fails with EINTR, which the
	code catches. However, the code fails to clear the
	r/w/e arrays (like poll3 does), which means it acts as
	if every descriptor had received all possible events.

Bug report and patch by Cesar Eduardo Barros