variable expansion in sdctld.conf

[Francis Dupont]

I think a variable expansion in sdctld.conf will be very useful,
i.e., "bitrate_up=$BR" will be replaced by "bitrate_up=1000000" when
the variable BR is set to 1000000 during sdctld.conf file reading.

So I have two questions:
 - is it a good idea?
 - if it is adopted, should be the variables command line arguments
  or simply environment variables?

Thanks

Francis Dupont <fdupont at isc.org>