Merge kern_ktrace.c:1.102, ktrace.h:1.30 from HEAD to RELENG_6:

Merge kern_ktrace.c:1.102, ktrace.h:1.30 from HEAD to RELENG_6:

  Replace ktr_buffer pointer in struct ktr_header with a ktr_unused
  intptr_t.  The buffer length needs to be written to disk as part
  of the trace log, but the kernel pointer for the buffer does not.
  Add a new ktr_buffer pointer to the kernel-only ktrace request
  structure to hold that pointer.  This frees up an integer in the
  ktrace record format that can be used to hold the threadid,
  although older ktrace files will have a garbage ktr_buffer field
  (or more accurately, a kernel pointer value).

  Space requested by:     davidxu