Added comedi_board_info which has been inspired by the original

Added comedi_board_info which has been inspired by the original
info in the demo directory. I've made it a bit more verbose
and also took into account that now the generic_timed command
takes more than one channel so that a user can experiment which
sampling rates are possible with different channel numbers.
I've also added an option to set the sampling rate for the generic_timed
command. That sould actually not be needed but I'm sure there
are drivers out there which do not correct the sampling rate
downwards automatically.