When the network library is no longer required (as of the first free source
drop, netlib is always required and cannot be broken out as a separate
component), a call is made to NET_ShutdownNetLib() to cleanup anything
netlib may have done.
Cleanup consists of aborting any urls waiting for load completion, destroying/freeing
any memory allocated (including the memory cache), closing the global history
database, and writing the cookies in memory to disk.