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