Add a function to reinit a decoder as is needed by PSI decoders.

The struct members ::p_current_valid, ::i_last_section_number and
::ap_sections[] array are used by all PSI section decoders. They are
moved to DVBPSI_DECODER_COMMON.

Implemented a function fifo_size() that returns the current size of the fifo.
The dvbinfo example program uses fifo_size() to detect if the total size of
buffers inside the fifo exceeds a certain threshold. The threshold is currently
defined at compile time.

Refactor out common code for checking PSI sections for PSI tables.

All PSI table check the same common things before accepting a section as
being a valid section. Centralize these checks into a single function.

Refactor dvbpsi_GatherPATSections().

The callback member of dvbpsi_decoder_t is used by dvbpsi_GatherXXSections()
functions. This patch makes the usage visisble.

Update doxygen information and fix linkage of howto-new-api.doxygen

The functions dvbpsi_GatherXSections() and dvbpsi_NewDemuxSubDecoder() take a
private decoder as argument. Change this argument from 'void *' to 'dvbpsi_decoder_t *'.
This makes its use more clear.

Use new utility functions to replace common code blocks. Moving
code to demux.c instead of repeating them in the subtable functions.
This reduces the amount of code in subtable files and reduces the risk
of introducing errors in future for this code.

Refactor common demux code structures into utility functions.
See the complete list below:
- rearrange arguments of dvbpsi_demux_new_cb_t callback
- define dvbpsi_demux_gather_cb_t callback type
- define dvbpsi_demux_detach_cb_t callback type
- new utility function dvbpsi_NewDemuxSubDecoder()
- new utility function dvbpsi_DeleteDemuxSubDecoder()
- new utility funciton dvbpsi_AttachDemuxSubDecoder()
- new utility function dvbpsi_DetachDemuxSubDecoder()

Renaming pf_callback to pf_gather to reflect its usage. It also
removes a typedef which was only used inside src/demux.c

Include config.h as first include file. It comes with some defines needed
for compilation.

Indentation cleanup.
Signed-off-by: Jean-Paul Saman <jean-paul.saman@m2x.nl>

Some readability improvements.
Signed-off-by: Jean-Paul Saman <jean-paul.saman@m2x.nl>

Slightly modified before applying.
Signed-off-by: Jean-Paul Saman <jean-paul.saman@m2x.nl>

The stdint.h header file was included twice.