MFC ath(4) potential memory disclosure fixes

[1] r327499: ath: fix memory disclosure from ath_btcoex_ioctl

The ath_btcoex_ioctl handler allocated a buffer without M_ZERO and
returned it to userland without writing to it.

The device has permissions only for root so this is not urgent, and the
fix can be MFCd and considered for a future EN.

[2] r327500: ath: fix possible memory disclosures in ioctl handlers

Apply the fix from r327499 to additional ioctl handlers.

Note: related fix in r327529 does not apply directly to stable/10 and
will be addressed in a followup commit.

Submitted by: Domagoj Stolfa <domagoj.stolfa@gmail.com> [1]
Reported by: Ilja van Sprundel <ivansprundel@ioactive.com> [1,2]
Reviewed by: adrian [1]
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix format size.

Add support for triggering spectral scan upon a channel reset/change.

This is intended to support reporting FFT results during active channel
scans, for users who would like to fiddle around with writing applications
that do both FFT visualisation _and_ AP scanning.

* add a new ioctl to enable/trigger spectral scan at channel change/reset;
* set do_spectral consistently if it's enabled, so a channel set/reset
will carry forth the correct PHY error configuration so frames
are actually received;
* for NICs that don't do spectral scan, don't bother checking the
spectral scan state on channel change/reset.

Tested:

* AR9280 - STA and scanning;
* AR5416 - STA, ensured that the SS code doesn't panic

Don't call the spectral methods for NICS that don't implement them.

Add a new (skeleton) spectral mode manager module.