[PATCH] Detect usage of dvb-scan data automatically

If /usr/share/dvb exists allready (from a dependancy or other package)
it is not needed to fetch the git tree, since it should be identical.

This should accompany https://github.com/tvheadend/tvheadend/commit/7a52ff434c8287d0f1a68d709cb41d0c4678c737#support/getmuxlist

We have talked about this in https://github.com/tvheadend/tvheadend/pull/168

So (maybe in a different form?) this should be used now. the dtv tables should be under active maintenance now (I am official maintainer of that specific git repo).