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

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Every path in a radix trie ends with a leaf or a NULL. By replacing
NULL (non-leaf) pointers with NULL leaves, there is a NULL test
removed from every iteration of an index-based search loop.

This speeds up radix trie searches by few percent. If there are any
radix tries that are not initialized with the init() function, but
instead depend on zeroing everything being proper initialization, this
will break those tries.

Reviewed by: alc, kib
Tested by: pho (as part of a larger change)
Differential Revision: https://reviews.freebsd.org/D41171

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

sys/sys: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

Add pctrie_init() and vm_radix_init() to initialize generic pctrie and
vm_radix trie.

Existing vm_radix_init() function is renamed to vm_radix_zinit().
Inlines moved out of the _ headers.

Reviewed by: alc, markj (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D11661

- Add a new general purpose path-compressed radix trie which can be used
with any structure containing a uint64_t index. The tree code
auto-generates type safe wrappers.
- Eliminate the buf splay and replace it with pctrie. This is not only
significantly faster with large files but also allows for the possibility
of shared locking.

Reviewed by: alc, attilio
Sponsored by: EMC / Isilon Storage Division