ringbuf: Initial implementation (with documentation and test)

Implementation is (believe) correct, subject to specification, but is
not optimal because the memory orderings are more aggressive than needed.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
gitignore: Update for autotools parallel test driver.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
messageq: Trivial reword of the brief description.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>