DVB improvements

Make DVB output simpler to use by encapsulating DVB-TXT packets in an mpeg transport stream.

This provides two new output formats ts and tsnpts to generate an encapsulated DVB-TXT stream with or without program clock reference and presentation timestamps respectively.

The PID of the teletext stream may be set using the --pid argument which accepts a decimal integer.

The previous un-encapsulated PES output format is removed in this release.