Make netmgr initialize and cleanup Winsock itself

Make netmgr initialize and cleanup Winsock itself

On Windows, WSAStartup() needs to be called to initialize Winsock before
any sockets are created or else socket() calls will return error code
10093 (WSANOTINITIALISED).  Since BIND's Network Manager is intended to
work as a reusable networking library, it should take care of calling
WSAStartup() - and its cleanup counterpart, WSACleanup() - itself rather
than relying on external code to do it.  Add the necessary WSAStartup()
and WSACleanup() calls to isc_nm_start() and isc_nm_destroy(),
respectively.

(cherry picked from commit 88f96faba872cfe7006c6de84759515bdf6e9c47)