introduce WEXT scan capabilities

Introduce scan capabilities to WEXT so that userspace can do intelligent
things with scan behavior such as handling hidden SSIDs more gracefully.
If the driver reports a specific scan capability, the driver must
respect the options specified in the iw_scan_req structure when handling
the SIOCSIWSCAN call, unless it's mode or state does not allow it to do
so, in which case it must return an error.

This version switches to Dave Kilroy's suggestion of claiming unused
padding space for the scan_capa field.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>