lib: 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

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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

libcasper: Constify cap_sysctl_limit_mib() mib parameter

No functional change. Minor API change that is nicer for consumers. ABI is
identical; the routine never needed to modify the pointed to value.

Reviewed by: emaste, markj
Differential Revision: https://reviews.freebsd.org/D24319

cap_sysctl: correct typo from r347534-ish

operation & ~limit where limit is a bool is clearly not what was intended,
given the line prior. Correct it to use the calculated mask for validation.

The cap_sysctl tests should now be functional again.

Add cap_sysctl(3) and cap_sysctlnametomib(3).

These complement cap_sysctlbyname(3) to provide a drop-in
replacement for the corresponding libc functions.

Also revise the libcap_sysctl limit interface to provide access
to sysctls by MIB, and to avoid direct manipulation of nvlists
by the caller.

Reviewed by: oshogbo
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17854

Introduce channel flags in libcasper.

Instead of passing flags (which describe a type of nvlist)
every send/recv we remember them in channel.
It's enough for use to extract them only during unwrap.
This simplify use of Casper.

Reviewed by: bruffer@, bcr@ (both man page)
Differential Revision: https://reviews.freebsd.org/D14196 (man page)

Add SPDX tags for libcasper(3) and services.

MFC after: 2 weeks

Add flags to the Casper services.

CASPER_SERVICE_STDIO - Casper will not close the first three descriptors (stdin,
stdout and stderr) this can be helpful for debugging.
CASPER_SERVICE_FD - Capser will not close all other descriptors, this can
be useful for a filesystem service.

Convert casperd(8) daemon to the libcasper.
After calling the cap_init(3) function Casper will fork from it's original
process, using pdfork(2). Forking from a process has a lot of advantages:
1. We have the same cwd as the original process.
2. The same uid, gid and groups.
3. The same MAC labels.
4. The same descriptor table.
5. The same routing table.
6. The same umask.
7. The same cpuset(1).
From now services are also in form of libraries.
We also removed libcapsicum at all and converts existing program using Casper
to new architecture.

Discussed with: pjd, jonathan, ed, drysdale@google.com, emaste
Partially reviewed by: drysdale@google.com, bdrewery
Approved by: pjd (mentor)
Differential Revision: https://reviews.freebsd.org/D4277