kernel - Adjust impossible conditions in kdmsg_state_*()

* kdmsg_state_reply() and kdmsg_state_result() require a non-NULL state.
  Assert the condition and remove the else clause that was broken in the
  first place.

Reported-by: Joris giovannang
pthread.3: add pthread barrier routines
pthread.3: Fix typo from previous commit.
arp: Dispatch arp cache expiration operation to netisr
kprintf.9: %D was removed, so don't mention it.
Fixing bugs in zero-checking path.
hammer2 - Add missing bqrelse()

* Add missing bqrelse() in hammer2_chain_load_async()

Reported-by: iostream
Merge remote-tracking branch 'crater/master' into hammer2_LZ4
hammer2 - Must clear CHAIN_INITIAL after updating device buffer

* We must clear the CHAIN_INITIAL flag after updating the device buffer
  to prevent reloads from believing that the data must be zero'd.

Reported-by: iostream
hammer2 - Add missing bzero()

* In hammer2_chain_load_async_callback() we have to bzero the buffer
  if the chain is still in the INITIAL state.
Merge remote-tracking branch 'crater/master' into hammer2_LZ4
hammer2 - Add missing bzero() (2)

* Fix type-o
Merge remote-tracking branch 'crater/master' into hammer2_LZ4
Trying to fix the bug.