Exec() has been renamed to Run() in keeping with the os/exec API.

Added func (*Session) Start(cmd string) which starts a remote process but unlike Run() doesn't wait for it to finish before returning.

Run() has been refactored to use Start internally. Its really just a refactoring, no new code but some extra functionality was won.

Also added func (*Session) Signal(sig signal) which sends a UNIX signal to a remote process. This is espcially useful in conjunction with Start() as the two allow you to start a remote process, monitor its stdout/stderr, and send it a TERM/HUP/etc signal when you want it to close.

R=dave, rsc, agl, bradfitz, n13m3y3r, gustavo
CC=golang-dev
https://golang.org/cl/5437058