Support SRT inputs.

Support SRT inputs.

FFmpeg can support SRT in VideoInput, but this goes beyond that;
the number of SRT inputs can be dynamic (they can fill any input
card slot), and they generally behave much more like regular input
cards than video inputs. SRT input is on by default (port 9710)
but can be disabled at runtime.

Due to licensing issues (e.g. Debian does not currently have a
suitable libsrt, as its libsrt links to OpenSSL), it is possible
to build without it.