NI m-series software calibration support

I've added support for the software ai/ao calibration on ni m-series 
boards.  The code hasn't been fully tested, it will probably be a little 
while before I get around to doing more with it, so I thought I'd let 
people know about it in case they want to try it or comment on it.  I do 
know that comedi_soft_calibrate works on a 6289 board.
If you want to try it, you'll need to get comedilib and comedi_calibrate 
from cvs.  comedi 0.7.73 is probably okay, although you won't be able to 
see the SDF_SOFT_CALIBRATED subdevice flag unless you get comedi from cvs 
too.  

comedi_soft_calibrate doesn't have any documentation yet, although you can 
run "comedi_soft_calibrate --help" to get the options.  By default, it 
will try to use /dev/comedi0 and write it's output file to the same 
directory comedi_calibrate uses by default.

The new functions in comedilib are comedi_get_softcal_converter(), 
comedi_get_hardcal_converter(), comedi_to_physical(), and 
comedi_from_physical().  comedi_parse_calibration_file() was also extended 
to read in the software calibration information.  I've added reference 
entries for them to the comedilib documentation.  You can just look at 
comedilib/doc/funcref if you don't want to build the docs.

-- 
Frank