Now read_index is the only function which creates new index nodes. All
other functions should provide a pointer for newly created node.

Program state can be changed from both threads now and
'state->prog_state' modification is protected with mutex

Two flag variables indicating current program state were replaced with
one variable holding a state defined in enum

Add new commands to enable and disable individual channels

Update status output format

Record jpegs from all channels

Convert some major members of the global struct to arrays. The app can
take frames from first channel.