This commit makes the following changes:

1. We remove the InProgress constant from the VM.

2. When we detect EISCONN as the result of a connect(2), we ignore it.

3. We properly read SO_ERROR from a socket when connecting in order to
   determine the actual result of connect(2).

4. We fix cloning of registered sockets.

== Removal of InProgress

Using the InProgress variant meant that the result register of
SocketConnect would not be set when a connect is in progress. This could
lead to unexpected results if one were to assume it was actually set.

== Using SO_ERROR

Socket::connect() in the VM now checks the value of SO_ERROR when a
connect(2) is reported as blocking. On at least Windows, the result of
connect(2) always appears to be EAGAIN/WSAEWOULDBLOCK, with the actual
error being stored in SO_ERROR. Not checking this value could result in
a connect getting stuck in the loop of: performing the connect ->
reported as blocking -> retrying later on -> repeat.

== Socket cloning

We also fix the cloning of sockets to make sure that the "registered"
flag is not cloned, as doing this could lead to errors when trying to
register a socket with a network poller.