Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r230972,r230980,r231559,r231565: Add MK_SOURCELESS build option, and
associated fixes.

Approved by: kib (mentor)

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