"concurrency" branch updated

[Ken Raeburn]

I’ve tried to get the concurrency branch updated with respect to master, 
including the handlerlist/catchlist merge and timerfd support. It seems to be 
working okay, passing the tests Tom checked it for it, and correctly running a 
few nice, simple tests that I tried. It also crashes on a not-so-nice, simple 
test I tried, but the old version did too. :-)

I dropped the changelogs. (Arguably maybe I should’ve pulled out just the 
branch-specific items and kept them?) I’ve tried to preserve the 
Windows-related changes, but not having a Windows box, I couldn’t test that 
part. I did some testing on Debian GNU/Linux and Mac OS X, but nothing fancy.

I haven’t done anything about addressing issues brought up a couple years back 
when this work was discussed here, but I’ve been starting to go back over some 
of those emails to see what’s needed.

Ken