num: implement CheckedDiv
auto merge of #8459 : thestinger/rust/checked, r=graydon
Derive Clone for IpAddr and SocketAddr
Do not execute the callback before cleaning up resources.
Instruct event loops to ignore SIGPIPE when constructed.

libuv does not always catch SIGPIPE.
Working homing UDP socket prototype.
Homed UDP sockets
Make IO thread-safe.

Each IO handle has a home event loop, which created it.
When a task wants to use an IO handle, it must first make sure it is on that home event loop.
It uses the scheduler handle in the IO handle to send itself there before starting the IO action.
Once the IO action completes, the task restores its previous home state.
If it is an AnySched task, then it will be executed on the new scheduler.
If it has a normal home, then it will return there before executing any more code after the IO action.