netmgr fixes needed for dispatch

- The read timer must always be stopped when reading stops.

- Read callbacks can now call isc_nm_read() again in TCP, TCPDNS and
  TLSDNS; previously this caused an assertion.

- The wrong failure code could be sent after a UDP recv failure because
  the if statements were in the wrong order. the check for a NULL
  address needs to be after the check for an error code, otherwise the
  result will always be set to ISC_R_EOF.

- When aborting a read or connect because the netmgr is shutting down,
  use ISC_R_SHUTTINGDOWN. (ISC_R_CANCELED is now reserved for when the
  read has been canceled by the caller.)

- A new function isc_nmhandle_timer_running() has been added enabling a
  callback to check whether the timer has been reset after processing a
  timeout.

- Incidental netmgr fix: always use isc__nm_closing() instead of
  referencing sock->mgr->closing directly

- Corrected a few comments that used outdated function names.

lib/isc/netmgr/netmgr-int.h

@@ -976,8 +976,8 @@ struct isc_nmsocket {
 	atomic_bool listening;
 	atomic_bool connecting;
 	atomic_bool connected;
-	bool accepting;
-	bool reading;
+	atomic_bool accepting;
+	atomic_bool reading;
 	isc_refcount_t references;
 
 	/*%