dahdi_cfg: Make -S option based on spannumber instead of order in config file.

If the config file has two spans defined:

  span=1,1,0,esf,b8zs
  bchan=1-23
  dchan=24
  echocanceller=mg2,1-23
  span=2,2,0,esf,b8zs
  bchan=25-47
  dchan=48

And you only want to configure span-2, you would need to pass -S 1 to
dahdi_cfg. Now make the -S option take the span number as assigned in the
config file.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>