The typical UNIX method for controlling long running process is to
send the process signals. Since this doesn't get you very far, various
ad-hoc, remote-control protocols have been used over time by programs
like Apache and BIND.

Implementing an SSH server means that Go code will have a standard,
secure way to do this in the future.

R=bradfitz, borman, dave, gustavo, dsymonds, r, adg, rsc, rogpeppe, lvd, kevlar, raul.san
CC=golang-dev
https://golang.org/cl/4962064