NI general-purpose counter changes

I've converted ni_660x.c and ni_mio_common.c to use ni_tio.c for their 
counter support.  The counters now each have their own subdevice with 3 
channels (count, load a register, and load b register).  The configuration 
instructions have changed also, they are more primitive now and closer to 
the hardware.  This allows a little more of the counters' cabailities to 
be exposed.  If there is a need for higher level configuration support, it 
might be added as functions in comedilib, or maybe put with the scxi stuff 
in a utility library.  There is still no support for buffered counting 
yet.  There are a couple demo programs added to the comedilib cvs, 
gpct_pulse_generator.c and gpct_simple_counting.c which call a few 
functions defined in demo/common.c, and use some new enums defined in 
comedi.h.  I've tested it with an m-series 6289, but I don't have access 
to any 660x boards.

-- 
Frank