IOCP.Winsock.Mswsock: define withPtrLen locally

I'm planning to split the network code out, so the network code can't depend on
windows-iocp internals.  This was the only point where the network modules
depended on an internal module of windows-iocp.
Remove network code; windows-iocp just has the I/O manager now

This will let me develop a fork of the network package that depends
on windows-iocp, so I can easily test anything on Hackage with it.
To do this, windows-iocp can't depend on the network package, since
the network package needs to depend on windows-iocp.
IOCP.Manager: add isManagerAvailable