sys: Automated cleanup of cdefs and other formatting

Apply the following automated changes to try to eliminate
no-longer-needed sys/cdefs.h includes as well as now-empty
blank lines in a row.

Remove /^#if.*\n#endif.*\n#include\s+<sys/cdefs.h>.*\n/
Remove /\n+#include\s+<sys/cdefs.h>.*\n+#if.*\n#endif.*\n+/
Remove /\n+#if.*\n#endif.*\n+/
Remove /^#if.*\n#endif.*\n/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/types.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/param.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/capsicum.h>/

Sponsored by: Netflix

sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

geom: clean up empty lines in .c and .h files

geom_label: Make glabel labels more trivial by separating the tasting
routines out.

While there, also simplify the creation of label paths a little bit
by requiring the / suffix for label directory prefixes (ld_dir renamed
to ld_dirprefix to indicate the change) and stop defining macros for
these when they are only used once.

Reviewed by: cem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25597

Squash error from geom by sizing ident strings to DISK_IDENT_SIZE.

Display attribute in future error strings and differentiate g_handleattr()
error messages for ease of debugging in the future.

"g_handleattr: md1 bio_length 24 strlen 31 -> EFAULT"

Reported by: swills
Reviewed by: imp cem avg
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14962

sys/geom: 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.

Comment typo fix.

Is aware of the importance of comments: dim

Fix the buffer-overflow-fixing fixes.

Pointy-hat to: me, for not realizing snprintf() is available in kernel.
Thanks to: jh, for bringing me the good news of snprintf(), Pawel Worach, for
noting that the panic can be provoked in i386 and not in amd64

Introduce glabel labels based on GEOM ident attributes. In this initial
implementation, error on the side of conservatism and only create labels
for GEOMs of classes DISK and MULTIPATH.

Discussed with: trasz
Approved by: silence from freebsd-geom@