lib: change controlling expressions in if/while to Boolean

Use `do { ... } while (false)' instead of `do { ... } while (0)'.
Use comparisons with zero instead of implicitly testing integers.
Use comparisons with NULL instead of implicitly testing pointers.
Use comparisons with NUL instead of implicitly testing plain chars.
Use `bool' instead of `int' to represent Boolean values.
Use `while (true)' instead of `while (1)' to express infinite loops.

Signed-off-by: frei tycho <tfrei@baumer.com>