Another refactoring. Changed 'socket' from being a factory function

Another refactoring.  Changed 'socket' from being a factory function
to being a new-style class, to be more similar to the socket class
in the _socket module; it is now the same as the _socketobject class.
Added __slots__.  Added docstrings, copied from the real socket class
where possible.

The _fileobject class is now also a new-style class with __slots__
(though without docstrings).  The mode, name, softspace, bufsize and
closed attributes are properly supported (closed as a property; name
as a class attributes; the softspace, mode and bufsize as slots).