[multipathd] interim support for reaping DM events waiter threads

[multipathd] interim support for reaping DM events waiter threads

It is sort of late in the game for redhat to be making kernel patches for the
next release, so here is an alternate way of getting to break out of the
waitevent pthread.

	Martin Marzinski, Redhat.

I merge the patch, mainly because some users will get stuck unkowing of
Ed's patch, and it doesn't get in the way when this kernel patch will be
merged.