lib.[ch], dvd-sector-copy.c: Publish the `buf' machinery as inline functions.

lib.[ch], dvd-sector-copy.c: Publish the `buf' machinery as inline functions.

Translate the macros into inline functions, and export them from
`lib.h', with some minor tweaking.

  * I've added `buf_putz', and fixed `buf_putc' to advance the output
    pointer as one would expect.

  * I've pulled the actual buffer extension into an out-of-line
    function.