Remove $FreeBSD$: one-line nroff pattern

Remove /^\.\\"\s*\$FreeBSD\$$\n/

Drop 'Set to' from most src.conf(5) knobs

The description is clearly what effect the knob has when set, so the
additional text was unnecessary.

Reviewed by: jhb, se
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29583

Add WITH_/WITHOUT_LLD knobs to enable the lld linker

Use this to control inclusion of the libllvm functionality required
by lld. Enable by default on arm64 and amd64, the two platforms where
lld is most usable for testing.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7713