Library - step one

- Added some definitions to the decoders_common_h so that if we no
longer import ccextractor.h, things don't fall over.
- Moved 2 definitions from decoders_common to 608.h, since they are only
used by 608 decoder and hence are not common.
- Created a 608 decoder settings variable in ccx_options to replace some
other variables and adjusted code accordingly. Variables in 608 decoder
settings are only used in 608 decoder.
- removed unused headers in 608.c