win32: handle /dev/zero and /dev/urandom in open and read functions

Currently /dev/zero is handled as a special case in dd.  Add hacks
to the open and read functions in mingw.c to handle the zero and
urandom devices.

- Opening /dev/zero or /dev/urandom actually opens the special
  Windows file 'nul' which behaves like /dev/null.  This allows
  manipulation of the file descriptor with things like seek and
  close

- When /dev/zero or /dev/urandom is opened the resulting file
  descriptor is stored and used to override the behaviour of read.

- No attempt is made to track duplicated file descriptors, so using
  these devices for redirections in the shell isn't going to work
  and won't be permitted.  (Could be, but won't.)

- Limited control of the special file descriptors is provided by
  allowing the internal variables to be changed.

- The numbers from /dev/urandom aren't very random.

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