libdvbv5: better handle ATSC/Annex B

libdvbv5: better handle ATSC/Annex B

As DVBv3 is confusing with regards to ATSC and ClearQAM (DVB-C
annex B), userpace apps also only differenciate between ATSC and
ClearQAM via modulation.

However, when using DVBv5, may be using the delivery system
in order to enforce one or the other.

In any case, the DVB API should clearly identify between ATSC
and ClearQAM.

So, make the API to better handle it, fixing the delivery
system if needed, when reading or write a file.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>