[socket] Enable Thread.Abort on blocking syscall

[socket] Enable Thread.Abort on blocking syscall

By installing interrupt token around the blocking syscalls, we make it possible to abort them with signals.

Previously, the only way to interrupt a blocking syscall, would have been to dispose of the SafeSocketHandle associated to the socket. But that make it impossible to abort the following case:
 - thread T1 accept a socket S and launch T2 (as a background thread) to receive on S
 - T2 block on receive
 - T1 (the main thread) quit, and the shutdown code try to abort T2

T2 would never be aborted, because S would never return from its blocking syscall, and that's because T2:
 - would still have a reference to S, making it impossible to call the finalizer on S
 - will not call Dispose on S before returning from the blocking syscall

This fixes the hang observed on System.Net.HttpWebRequestTest.GetRequestStream with the cooperative GC.