sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

subr_physmem: Fix userspace build

Include stdbool.h in userspace configurations. For the kernel builds we
get it from sys/types.h, but bool isn't defined there for non-kernel
builds and this otherwise kernel-only file is used for the physmem test
suite.

Fixes: deb1e3b71998
Sponsored by: Netflix

physmem: Add physmem_excluded to query if a region is excluded

In order to safely reuse excluded memory when it's reserved for special
purpose, we need to test whether or not the memory has been reserved
early in boot. physmem_excluded will return true when the entire range
is excluded, false otherwise.

Sponsored by: Netflix

Convert arm's physmem interface to MI code

The arm_physmem interface found in arm's MD code provides a convenient
set of routines for adding/excluding physical memory regions and
initializing important kernel globals such as Maxmem, realmem,
phys_avail[], and dump_avail[]. It is especially convenient for FDT
systems, since we can use FDT parsing functions and pass the result
directly to one of these physmem routines. This interface is already in
use on arm and arm64, and can be used to simplify this early
initialization on RISC-V as well.

This requires only a couple trivial changes:
- Move arm_physmem_kernel_addr to arm/machdep.c. It is unused on arm64,
and manipulated entirely in arm MD code.
- Convert arm32_btop/arm64_btop to atop. This is equivalently defined
on all architectures.
- Drop the "arm" prefix.

Reviewed by: manu, emaste ("looks reasonable")
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24153