ShutdownAndWait mirrors the net/http Shutdown API allowing the user to
wait until the server is idle or until the provided context is canceled.

And update README example to fall in line.

(1) change golang to Go (preferred brand name), (2) use module instead
of library, and (3) style links.

Help the syntax highlighter out.

To keep ourselves honest (;

This may be the most complete initial commit in recent history. sshutil
includes a README, a fully functional (to the channel/session boundary)
server implementation, a few tests, and example programs.