move findfreeiocb function into its own object file
Merge pull request #16 from groessler/something_to_pull

move findfreeiocb function into its own object file
No delimiter comment between includes and first segment.
Have __PLUS4__ imply __C16__.
Made EXTZP segment option for modules.

This allows to remove the "add empty EXTZP segement to avoid warnings"
approach.
Don't save and restore the zero page locations used.
Saves 19 bytes code (in the exe file) and reduces memory footprint
of the program by 45 bytes (code & bss).
Merge pull request #17 from groessler/something_to_pull

Atari targets: Don't save and restore the zero page locations used.