ck_pflock: Phase-fair read-write lock implementation.

John Wittrock has contributed a phase-fair reader-writer
lock implementation. These locks allow phase fairness
guarantees between readers and writers. This work includes
additional changes and clean-up.

Follow-up work is expected.

Thanks to John Wittrock for patches and Professor Gabriel
Parmer (http://www.seas.gwu.edu/~gparmer/) for advising.