loader: provide a features table for binary compatibility advertisement

liblua now provides a loader.has_feature() function to probe the loader
binary for features advertised. name => desc mappings are provided in
loader.features to get a list of all of the features loader *can*
support. core.hasFeature is provided as a shim to loader.has_feature
so that individual consumers don't need to think about the logic of the
loader module not providing has_feature; we know that means the feature
isn't enabled.

The first consumer of this will be EARLY_ACPI to advertise that the
loader binary probes for ACPI presence before the interpreter has
started, so that we know whether we can trust the presence of acpi.rsdp
as relatively authoritative. In general, it's intended to be used to
avoid breaking new scripts on older loaders within reason.

This will be used in lua as `core.hasFeature("EARLY_ACPI")`, while the
C bits of loader will `feature_enable(FEATURE_EARLY_ACPI)`.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D42695

stand: Retire setting hw.ata.wc: it doesn't exist.

hw.ata.wc was disconnected as part ot the 2013 cam-ification of ata. No
need to continue setting it. It's been unused in FreeBSD 10.x and newer.

Sponsored by: Netflix

stand: Retire setting hw.eisa_slots.

When the eisa code was removed in 2017, prior to the stable/12 branch,
setting hw.eisa_slots became a nop. The oldest supported branch doesn't
have eisa at all. The need to set it manually on boot disappeared
largely by 2000...

Sponsored by: Netflix

loader: improve lua ACPI detection and handling

This is a follow-up patch to https://reviews.freebsd.org/D42459
that modifies the loader lua to use the correct loader variables
for determining ACPI availability.

This also fixes a bug where ACPI can be inadvertently disabled when
setting System Defaults at the loader menu.

Reviewed by: imp, kevans
Approved by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42483

Remove $FreeBSD$: two-line nroff pattern

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

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

Eliminate zfsloader man page.

Remove all cross references to zfsloader.8 and /boot/zfsloader.
Move ZFS specific info into loader.8.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D16361

Remove "All Rights Reserved" on files that I hold sole copyright on

See r333391 for more detail; in summary: it holds no weight and may be
removed.

core.lua(8): Update to reflect recently added function clearCachedKernels

core.lua(8): Add missing note about core.KEYSTR_CSI

Add core.lua(8), but do not add to distribution

Distribution will be done after all of the lualoader manpages are created.

Reviewed by: rpokala
Differential Revision: https://reviews.freebsd.org/D14479