ssh.ParsePublicKey parses a certificate in ssh wire formate. The
ssh.ParseAuthorizedKey method can parse the human-serialized version of
an ssh certificate as is used when writing data to the filesystem.

* Add utility funcs to load ssh certs.
* Rename key loading funcs to mirror the ssh package.
* Replace the example server key with an unencrypted key.

Add slightly more description.

The example demonstrates a graceful shutdown and is more akin to what
people should use in real applications.

Prevents a panic when a different goroutine untracks a listener or
connection while the server is being shutdown/closed.

Namely, clarify the different parts of the ssh protocol architecture and
more specifically reference them in wording.

Rename GlobalRequest to Request since it's global nature is implicit by
the fact that it is a method on Server, and explicit by the fact that it
now receives a reference to the associated connection when it is called.

Group related concepts more closely. Add more color to the Server
introduction.

Don't trigger initialization of the whole server when adding channel and
request handlers. We only want to initialize the server right before it
begins serving.

Convention is to use package name.

* Fix redundant log previx
* Add more info to channel accept/reject logs

This provides a little extra info to clients.

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.