version 2.6.3.4 - #300

The symptom was that `cabal sdist` would rightly fail with

```
Building source dist for network-2.6.3.2...
cabal: can't find include file HsNetworkConfig.h
```

Instead, the proper way is to inject autogenerated C header files
via .buildinfo (which is generated at `configure`-time).

We want to test network with the verison of `cabal-install` that is most
likely to be found in the wild. This commit switches the required builds
to run with `cabal-install-2.0` and includes a failable pass with
`cabal-install-head` and `ghc-head`.

* Fix finalizer getting triggered in ghc 8.2.2

GHC 8.2.2 more aggresively reclaims `Weak`. This causes the socket to be
closed in `accept` before `c_accept` is called. Utilizing `withMVar`
allows the reference to be held until `c_accept` has been called.

* Update variable for readability.

It is common to close a file descriptor after sending it, however it is
not a required part of the work flow. There are uses for not closing a
file descriptor. Closing the file descriptor is too high level of a
decision.