Make std.stdio.File 'package' accessible

This is required in the new std.process.
StdioException: No system error msg when errno==0
Renamed File.isPipe to isPopened

Now that std.process will support pipe creation, the name isPipe will
only cause confusion, as it will in fact be false for "normal" anonymous
pipes.  It is only true for files opened by popen(), and its purpose is
simply to ensure they are closed with pclose().
std.process redesigned

This is a complete redesign of std.process. It has passed the community
review and is now ready for inclusion in Phobos master.

Review and vote:
http://forum.dlang.org/thread/gclsbrghhjitnfderoos@forum.dlang.org
http://forum.dlang.org/thread/mgstnugckomjpvdgpudz@forum.dlang.org