From: William Lee Irwin III <wli@holomorphy.com>

This patch implements wake-one semantics for page wakeups in a single step.
Discrimination between distinct pages is achieved by passing the page to the
wakeup function, which compares it to a pointer in its own on-stack structure
containing the waitqueue element and the page.  Bit discrimination is achieved
by storing the bit number in that same structure and testing the bit in the
wakeup function.  Wake-one semantics are achieved by using WQ_FLAG_EXCLUSIVE
in the codepaths waiting to acquire the bit for mutual exclusion.