This CL inverts the direction of the Stdin/out/err members of the
Session struct so they reflect the API of the exec.Cmd. In doing so
it borrows heavily from the exec package.

Additionally Shell now returns immediately, wait for completion using
Wait. Exec calls Wait internally and so blocks until the remote
command is complete.

Credit to Gustavo Niemeyer for the impetus for this CL.

R=rsc, agl, n13m3y3r, huin, bradfitz
CC=cw, golang-dev
https://golang.org/cl/5322055