Add a --dtshd option

The spdif decoder was hardcoded to assume that the spdif output is
capable of accepting high (>1.5Mbps) bitrates. While this is true
for modern HDMI spdif interfaces, the original coax/toslink system
cannot deal with this and will fail to work.

This patch adds an option --dtshd which can be enabled if you use
a DTS-capable receiver behind a HDMI link.