Target informational, not standards-track

The normative requirements are almost entirely already present
in RFC 2743.  Future revisions will indicate where the new recommendations
for behavior are introduced.

Remove RFC 2119 language as a result.
Add a little bit to the introduction
Note 'extra tokens'

Context-deletion and feature-not-implemented are mentioned
as examples.
Note that only creds must be fixed

RFC 2743 is a little strange and only requires that the credentials
handle remain fixed throughout subsequent calls in the loop, even
though there are other parameters which should probably also
remain fixed.  Call out this nit where relevant.
Allow empty tokens during negotiation

They have been seen in the wild, so it's better to accept them
than rigidly follow the spec.
A couple more places for Process_context_token

Might be useful here, but might still be error.
Missing </t>
Add a section for sample code

Sample code yet to come.
Add sample code for the loop

Use pipes for communication, and fork off a process so that
we run both initiator and acceptor.

The idea is to have the sample code be compilable as-is, with no
dangling references.  My testing should get this to be runnable, too,
though that will require some setting of the target name which will
not end up in the spec.

The native OS X GSSAPI library also doesn't expose a useful
GSS_C_NT_USER_NAME without going through contortions -- the header
in /usr/include/gssapi/ declares a symbol by that name but the
header from the actual implementation, in
/System/Library/Frameworks/GSS.Framework/Headers/ , has that
as a preprocessor macro.

This commit builds but does not run.