Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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.

Don't call setgrent() in an unportable way.

For FreeBSD 12, I'm considering updating setgrent() to have a function
prototype that conforms to POSIX. FreeBSD seems to be the only operating
system that lets setgrent() return an integer. It's also inconsistent
with setpwent().

It looks like our libcasper depends on setgrent() returning an integer.
Get rid of that.

Reviewed by: oshogbo
Differential Revision: https://reviews.freebsd.org/D6659

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