Connect mitigations(7) to the build

Also add some cross references.

Sponsored by: The FreeBSD Foundation

Remove $FreeBSD$: two-line nroff pattern

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

elfctl: update man page example for 'no' prefix

Reported by: Mark Millard on freebsd-current@
Fixes: c763f99d11fd ("elfctl: prefix disable flags with "no"")
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

elfctl: allow features to be specified by value

This will allow elfctl on older releases to set bits that are not yet
known there, so that the binary will have the correct settings applied
if run on a later FreeBSD version.

PR: 252629 (related)
Suggested by: kib
Reviewed by: gbe (manpage, earlier), kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28284

elftcl: add -i flag to ignore unknown flags

This may allow an identical elfctl invocation to be used on multiple
FreeBSD versions, with features not implemented on older releases being
silently ignored.

PR: 252629 (related)
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28130

elfctl: update earliest FreeBSD revision to 12.2

elfctl has been merged to stable/12, and so will appear in FreeBSD 12.2

Reported by: Dewayne Geraghty
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

Move ELF feature note tool to usr.bin/elfctl

elfctl is a tool for modifying the NT_FREEBSD_FEATURE_CTL ELF note,
which contains a set of flags for enabling or disabling vulnerability
mitigations and other features.

Reviewed by: csjp, kib
MFC after: 2 weeks
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23910