XSVF improvements:

 - Layer parts of XSVF directly over SVF, calling svf_add_statemove()
   instead of expecting jtag_add_statemove() to conform to the SVF/XSVF
   requirements (which it doesn't).

   This should improve XSTATE handling a lot; it removes most users of
   jtag_add_statemove(), and the comments about how it should really do
   what svf_add_statemove() does.

 - Update XSTATE logic to be a closer match to the XSVF spec.  The main
   open issue here is (still) that this implementation doesn't know how
   to build and submit paths from single-state transitions ... but now
   it will report that error case.

 - Update the User's Guide to mention the two utility scripts for
   working with XSVF, and to mention the five extension opcodes.

Handling of state transition paths is, overall, still a mess.  I think
they should all be specified as paths not unlike SVF uses, and compiled
to the bitstrings later ... so that we can actually make sense of the
paths.  (And see the extra clocks, detours through RUN, etc.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>