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

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

Add MK_SOURCELESS build option. Setting MK_SOURCELESS to "no" will disable
kernel modules that include binary-only code.

More fine-grained control is provided via MK_SOURCELESS_HOST (for native code
that runs on host CPU) and MK_SOURCELESS_UCODE (for microcode).

Reviewed by: julian, delphij, freebsd-arch
Approved by: kib (mentor)
MFC after: 2 weeks