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

src.conf(5): Fix some mandoc issues in source files

- new sentence, new line
- blank line in fill mode

Event: September 2020 Bugathon
MFC after: 1 week

Create MK_LOADER_VERBOSE and connect it to ELF_VERBOSE in the loader
code.

PR: 18498
Submitted by: mellon at pobox dot com