sleep(9), sleepqueue(9): const'ify wchan pointers

sleep(9), sleepqueue(9): const'ify wchan pointers

_sleep(9), wakeup(9), sleepqueue(9), et al do not dereference or modify the
channel pointers provided in any way; they are merely used as intptrs into a
dictionary structure to match waiters with wakers.  Correctly annotate this
such that _sleep() and wakeup() may be used on const pointers without
invoking ugly patterns like __DECONST().  Plumb const through all of the
underlying sleepqueue bits.

No functional change.

Reviewed by:	rlibby
Discussed with:	kib, markj
Differential Revision:	https://reviews.freebsd.org/D22914