- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

Approved by: re (implicit)
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

Extend dim's hack from r228978: not only clang but gcc on non-x86 platforms
warns about unused variables in this code, so always add -Wno-unused to
the warning flags. Why gcc on x86 *doesn't* warn about this, I will never
know. The code itself should probably be fixed at some point.

Don't try to build Linux compatibility stuff on platforms without
COMPAT_LINUX.

For several files in sys/dev/drm, disable -Wunused-value when building
with clang. There are several macros in these files that return values,
and in some cases nothing is done with them, but it is completely
harmless. For some other files, also disable -Wconstant-conversion,
since that triggers a false positive with the DMA_BIT_MASK() macro.

MFC after: 1 week

Welcome drm support for VIA unichrome chips.

MFC after: 2 weeks