Introduce a class to manage the lifetime of a socket - when the object
that contains the socket goes out of scope, the underlying socket will
be closed.
In addition, the new `Sock` class has a `Send()`, `Recv()` and `Wait()`
methods that can be overridden by unit tests to mock the socket
operations.
The `Wait()` method also hides the
`#ifdef USE_POLL poll() #else select() #endif` technique from higher
level code.
Move `CloseSocket()` (and `NetworkErrorString()` which it uses) from
`netbase.{h,cpp}` to newly added `src/util/sock.{h,cpp}`.
This is necessary in order to use `CloseSocket()` from a newly
introduced Sock class (which will live in `src/util/sock.{h,cpp}`).
`sock.{h,cpp}` cannot depend on netbase because netbase will depend
on it.
