Sync with FreeBSD CURRENT.

Sync with FreeBSD CURRENT.
Important changes:
- Ignore EINVAL from wi_getval for optional values to increase support for
  different drivers.
- Generally improved error handling.
- Better output and support for the generic 802.11 layer.
- Warnings output deprecated options which are supported by ifconfig,
  this will be removed at some time.

Additional changes:
- print "No stations" instead of "0 stations:"
- Remove 'Z', the zeroing of the signal cache is not supported by the
  newer driver versions anyway.
- Add WARNS?= 6 to Makefile.