DVB API

[Tony Houghton]

I've kind of volunteered to bring the dvb plugin (from "bad") up-to-date
to support DVB-S2 and DVB-T2
<https://bugzilla.gnome.org/show_bug.cgi?id=654479>. This requires
dvbsrc to use the linuxtv DVB API version 5 ("s2api"). In an earlier
effort I kept support for version 3, detecting whether 5 was avaliable
at compile time. The code will be easier to maintain long-term if I drop
support for API 3 altogether, especially as v5 uses properties to set
parameters instead of a monolithic struct, so I could just map (most of)
them to GObject properties.

DVB API 5 has been available since kernel 2.6.28 so is it OK to drop
suport for earlier versions in GStreamer 0.11/1.0?