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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

Move SMR pointer type definition and access macros to smr_types.h.

The intent is to provide a header that can be included by other headers
without introducing too much pollution. smr.h depends on various
headers and will likely grow over time, but is less likely to be
required by system headers.

Rename SMR_TYPE_DECLARE() to SMR_POINTER():
- One might use SMR to protect more than just pointers; it
could be used for resizeable arrays, for example, so TYPE seems too
generic.
- It is useful to be able to define anonymous SMR-protected pointer
types and the _DECLARE suffix makes that look wrong.

Reviewed by: jeff, mjg, rlibby
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23988