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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

lib: libnetmap: add missing copyright headers

MFC after: 2 weeks

lib: add libnetmap

This changeset introduces the new libnetmap library for writing
netmap applications.
Before libnetmap, applications could either use the kernel API
directly (e.g. NIOCREGIF/NIOCCTRL) or the simple header-only-library
netmap_user.h (e.g. nm_open(), nm_close(), nm_mmap() etc.)

The new library offers more functionalities than netmap_user.h:
- Support for complex netmap options, such as external memory
allocators or per-buffer offsets. This opens the way to future
extensions.
- More flexibility in the netmap port bind options, such as
non-numeric names for pipes, or the ability to specify the netmap
allocator that must be used for a given port.
- Automatic tracking of the netmap memory regions in use across the
open ports.

At the moment there is no man page, but the libnetmap.h header file
has in-depth documentation.

Reviewed by: hrs
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D26171