Refactor tidc/monc to better share code

Refactor tidc/monc to better share code

  * Implement minimal decoding of monitoring responses
  * Add tr_gss_client.[ch] to house GSS req/resp message exchange
  * Always use 'payload' as the key for MON_RESP payload, don't name it
    after the command that it is responding to
  * Use better reference count behavior for MON_RESP payload
  * Move typedefs out of mon_internal.h to mon.h to avoid cyclic header
    dependencies
  * Fix some minor integer type mismatches in option parser
  * Update various test programs to use extra argument to
    tr_msg_(en/de)code methods