Fix 12815: Add std.stdio.stdnull

There has been some interest in a cross-platform null device file (e.g. /dev/null), see issue 12815 (linked at bottom) and http://forum.dlang.org/thread/uwmeitsotcfpqkjxdsgl@forum.dlang.org

This implementation adds a `stdnull` File to std.stdio in the same manner as stdin, stdout, and stderr.

Discussion: This implements for Posix and Windows--does D currently target a platform which is not covered by these versions?  See wikipedia article here: http://en.wikipedia.org/wiki//dev/null

https://issues.dlang.org/show_bug.cgi?id=12815