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

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

[skip ci] update copyright headers in fusefs files

Sponsored by: The FreeBSD Foundation

fusefs: convert statistical sysctls to use counter(9)

counter(9) is more performant than using atomic instructions to update
sysctls that just report statistics to userland.

Sponsored by: The FreeBSD Foundation

fusefs: reap dead code

Sponsored by: The FreeBSD Foundation

Rename fuse(4) to fusefs(4)

This makes it more consistent with other filesystems, which all end in "fs",
and more consistent with its mount helper, which is already named
"mount_fusefs".

Reviewed by: cem, rgrimes
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19649

fuse(4): remove more debugging printfs

I missed these in r344664. They're basically useless because they can only
be controlled at compile-time. Also, de-inline fuse_internal_cache_attrs.
It's big enough to be a regular function, and this way it gets a dtrace FBT
probe.

Sponsored by: The FreeBSD Foundation

Fuse: whitespace and style(9) cleanup

Take a pass through fixing some of the most egregious whitespace issues in
fs/fuse. Also fix some style(9) warts while here. Not 100% cleaned up, but
somewhat less painful to look at and edit.

No functional change.

sys: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 3-Clause license.

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.

Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.

Import a FreeBSD port of the FUSE Linux module.
This has been developed during 2 summer of code mandates and being revived
by gnn recently.
The functionality in this commit mirrors entirely content of fusefs-kmod
port, which doesn't need to be installed anymore for -CURRENT setups.

In order to get some sparse technical notes, please refer to:
http://lists.freebsd.org/pipermail/freebsd-fs/2012-March/013876.html

or to the project branch:
svn://svn.freebsd.org/base/projects/fuse/

which also contains granular history of changes happened during port
refinements. This commit does not came from the branch reintegration
itself because it seems svn is not behaving properly for this functionaly
at the moment.

Partly Sponsored by: Google, Summer of Code program 2005, 2011
Originally submitted by: ilya, Csaba Henk <csaba-ml AT creo DOT hu >
In collabouration with: pho
Tested by: flo, gnn, Gustau Perez,
Kevin Oberman <rkoberman AT gmail DOT com>
MFC after: 2 months