This removes exec.Run and replaces exec.Cmd with a
new implementation. The new exec.Cmd represents
both a currently-running command and also a command
being prepared. It has a good zero value.

You can Start + Wait on a Cmd, or simply Run it.
Start (and Run) deal with copying stdout, stdin,
and stderr between the Cmd's io.Readers and
io.Writers.

There are convenience methods to capture a command's
stdout and/or stderr.

R=r, n13m3y3r, rsc, gustavo, alex.brainman, dsymonds, r, adg, duzy.chan, mike.rosset, kevlar
CC=golang-dev
https://golang.org/cl/4552052