Replace ktr_buffer pointer in struct ktr_header with a ktr_unused

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).

MFC after:		2 weeks
Space requested by:	davidxu