tvhlog: performance improvement when trace/debug is disabled.

The processing performed in the hexdump routine, commonly used in
SI table trace output. Was having a significant impact on performance
as it was not pre-processing the config to see if trace was even
enabled before continuing to build up the (large and frequent) hex
strings.
tvhlog: ensure the logging Q is bounded in size, else it can become a leak

If processing because so high that the log queue becomes swamped we can
end up with a sort of avalanche scenario where everything gradually slows
down and the size of the buffer continues to grow. The main culprits of
this have been eliminated at source, however this is still a useful
safe guard.
subscriptions: corrections to the way ota muxes are unlinked
opentv: updated to new APIs

This uses a significantly simpler setup, table is processed in a
linear fashion.
mpegts: improvements to DVB SI processing

I've made the table extraid 64 bits, as some entries require 32+X bits
so a 32-bit field wasn't enough.

Added callback for registering destruction (for dealloc'ing memory
alloc'd for opaque).

Added method of registering desciptor tag handlers, saves duplicating
BAT processing used for OpenTV, which uses 1 custom descriptor.

Improved the table completion detection and reduced level of debug
that could swamp tvhlog queue.
epggrab: corrections to the way the whole ota mux system works
epggrab: sort out configuration for epggrab ota muxes