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
Deprecate xformat and xsformat.

They were intended to be temporary, and now format and sformat use their
exact implementations and have for 6 months (though for some reason, xformat
and xsformat weren't turned into aliases when that happened, which I fixed
in this commit as well).
Undocument some deprecated functions.

They were marked for removal in the docs (many of them were intended to
be removed in January), so now they've been removed from the docs and
marked for removal from the code in November.
Removed some undocumented, deprecated functions from std.algorithm.

They were scheduled to be removed in January 2013 but weren't, so
they've been around as undocumented, and deprecated for a while now.
Merge pull request #1285 from jmdavis/deprecated

Move some deprecations along
Merge pull request #1272 from kyllingstad/std-process

std.process redesigned and reviewed
xformat -> format
Merge pull request #1286 from NilsBossung/xformat

remove the last traces of xformat