[patch] Make board_info display subdevice flags

The attached patch adds the subdevices' flags to board_info's output. 
Here is a sample of the new output...

overall info:
   version code: 0x000746
   driver name: ni_pcimio
   board name: pxi-6031e
   number of subdevices: 11
subdevice 0:
   type: 1 (analog input)
   flags: 0x01f11000        <--- This is the new info!
   number of channels: 64
   max data value: 65535
[etc.]

The patch also modifies the docs for comedi_get_subdevice_flags() to 
display the hex values in the table using 8 hex digits with leading 
zeros if needed.  This makes it much easier (IMHO) to tell which bits 
correspond to which flags.  Compare...

SDF_READABLE 0x10000
SDF_GROUND   0x100000

...to...

SDF_READABLE 0x00010000
SDF_GROUND   0x00100000

...when looking at flags like

flags: 0x02121000

Just my opinion.  I suppose getting the table to right align the hex 
values would help, but always using eight gets the values both right and 
left aligned.

Finally, this patch also cause board_info to display a more detailed 
error message if it can't open the device (useful for determining if 
it's an existence problem or a permissions problem).

Dave