Use a per-RX-queue deferred list, rather than a single deferred list for
both queues.

Since ath_rx_pkt() does multi-mbuf frame recombining based on the RX queue,
this needs to occur.

Tested:

* AR9380 (XB112), hostap mode
- Document that sem_wait() can fail with EINTR if it is interrupted by a
  signal.
- Fix the old ksem implementation for POSIX semaphores to not restart
  sem_wait() or sem_timedwait() if interrupted by a signal.

MFC after:	1 week
mdoc: remove superfluous paragraph macro.
Implement a utility function to return the current TX power cap for
the given node.

This takes into account the per-node cap, the ic cap and the
per-channel regulatory caps.

This is designed to replace references to ni_txpower in various net80211
drivers - ni_txpower doesn't necessarily reflect the actual cap for
the given node (eg if the node has the default value of 50dBm (100) and
the administrator has manually configured a lower TX power.)