I've collected in this patch a few small fixes for dvb-apps

1) in libdvbapi a new enum to support DMX_OUT_TSDEMUX_TAP

2) in libdvbapi a change in a comment where it is stated that the dvr can be opened more that once
in readonly. It can only be opened once.

3) tzap: removed the ioctl call to DMX_SET_BUFFER_SIZE on the dvr. This calls shrinks the size of
the buffer, from about 2MB (#define DVR_BUFFER_SIZE (10*188*1024) in dmxdev.h) to 1MB. It only
matters once the PATCH 2/3 enables DMX_SET_BUFFER_SIZE on the dvr.
I think the writer of the code wanted a bigger buffer, so it is pointless to reduce it.

Let me know if I should post 3 separate patches.

I plan to send an other patch to change gnutv to be more robust with slow writes and to support
DMX_SET_BUFFER_SIZE.

Andrea