usr.bin: 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/

users(1): Use capsicum helpers for users

In doing so also fix the libcasper.h header to work in C++.

Renumber copyright clause 4

Renumber cluase 4 to 3, per what everybody else did when BSD granted
them permission to remove clause 3. My insistance on keeping the same
numbering for legal reasons is too pedantic, so give up on that point.

Submitted by: Jan Schaumann <jschauma@stevens.edu>
Pull Request: https://github.com/freebsd/freebsd/pull/96

style(9) fix

Submitted by: jmallett

Capsicum: add capability mode to users binary

Submitted by: Tyler Littlefield <tyler@tysdomain.com>
Reviewed by: cem, oshogbo
Differential Revision: https://reviews.freebsd.org/D9046

Fix a couple of style nits.

- Use set instead of std::set, to be consistent with the rest of the file.
- Remove return (0); it's not required.
- Add a dash at the beginning of the copyright, per style(9).

Don't use auto, as we also need to support GCC 4.2.

Let users(1) use an std::set, instead of std::{vector,sort,unique}.

Reviewed by: gahr

Reimplements users(1) in C++.

This reduces the lines of code by roughly 50% (not counting the COPYRIGHT
header) and makes it more readable by using standard algorithms.

Approved by: bapt