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

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

The AAC_COMPAT_LINUX option was really annoying, since it made the
aac driver dependent on the linux emulation module. This was
especially bad for the release engineers who tried to move the
aac driver from the kernel onto the drivers floppy. The linux
compat bits for this driver are now in their own driver, aac_linux.
It can be loaded as a module or compiled into the kernel. For
the latter case, the AAC_COMPAT_LINUX option is needed, along with
the COMPAT_LINUX option.

I've tested this in every configuration I can think of. This is an
MFC candidate for 4.7.

Idea from: rwatson
MFC after: 3 days