MFV CK@r336629: Import CK as of commit 1c1f9901c2dea7a883342cd03d3906a1bc482583

This adds CK_SLIST_INSERT_PREVPTR and CK_SLIST_REMOVE_PREVPTR macros
as well as ck_pr_dec_is_zero family of functions.

MFC after: 3 weeks