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.
zero out token length when freeing

We deallocate the storage for the token that was locally allocated
by receive_token(); be careful and zero the length as well.
Back to strong flags

Require two calls to init_sec_context.
Remove some unneeded printfs
Helper for cleanup
Some cleanup and add comments

Be more pedantic about resource deallocation.
Helpers cannot cause program exist, as fully compliant applications
must release GSS resources before exiting (such resources are permitted
to be non-local to the process).
Move globals up a level

Make the initiator/acceptor implementations take file descriptors
to read/write on, instead of hardcoding the global pipe file descriptors.
Give a more concrete example of a target name

Leave it commented, though, as it won't work as-is.
Include anonymous support