A bunch of changes as the command line now works.

A bunch of changes as the command line now works.

* Move the main processing into a shared library
* Add processing in main() to read input, call the library commands,
  and output
* Correct copy-and-paste error in GSSNameCache that precluded it from
  being used along with GSSContextCache
* GSSAcquireCred now honors the desired_name parameter
* GSSCommand now an abstract class, with pure virtual methods
* GSSCreateSecContextCommand::execute() now uses datamodel objects
  instead of raw GSS types
* GSSCreateSecContextCommand now accepts cached names and contexts
  instead of raw GSS types
* GSSName operator= now duplicates GSS name instead of copying the
  data out of the other GSSName object, thus rendering their
  destruction independent
* GSSCreateSecContext now defaults to using the Moonshot mechanism
* Add a description of the JSON protocol for json_gssapi
* Clean up some of the compiler warnings about casting the result
  of rand() into a pointer