Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

cap_dns tests: Convert to ATF, avoid failing when lookups don't resolve

The cap_dns tests require Internet access. Currently they fail when
that's not available, which for CI purposes is undesirable. Let's
instead skip the tests if none of the non-casper name/addr lookups
succeed.

To that end:
- Convert the tests to ATF so that skipping is easier to implement.
- Break up the tests into separate test cases.
- If one of the system (i.e., non-casper) lookup functions fails, skip
the test if all of them failed, otherwise fail the tests, since
partial failure indicates something is flaky and deserves a closer
look.

Reviewed by: oshogbo
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39241

build: provide a default WARNS for all in-tree builds

The current default is provided in various Makefile.inc in some top-level
directories and covers a good portion of the tree, but doesn't cover parts
of the build a little deeper (e.g. libcasper).

Provide a default in src.sys.mk and set WARNS to it in bsd.sys.mk if that
variable is defined. This lets us relatively cleanly provide a default WARNS
no matter where you're building in the src tree without breaking things
outside of the tree.

Crunchgen has been updated as a bootstrap tool to work on this change
because it needs r365605 at a minimum to succeed. The cleanup necessary to
successfully walk over this change on WITHOUT_CLEAN builds has been added.

There is a supplemental project to this to list all of the warnings that are
encountered when the environment has WARNS=6 NO_WERROR=yes:
https://warns.kevans.dev -- this project will hopefully eventually go away
in favor of CI doing a much better job than it.

Reviewed by: emaste, brooks, ngie (all earlier version)
Reviewed by: emaste, arichardson (depend-cleanup.sh change)
Differential Revision: https://reviews.freebsd.org/D26455

Build service tests with Casper support.

Introduce caspermocks.

The idea behinds mocks is that we don't need to ifdef a lot of code in
tools itself but those defines are hidden in the casper library.
Right now the mocks are implemented as define/inlines functions.
There was a very long discussion how this should be implemented.
This approach has some advantages like we don't need to link to any additional
libraries. Unfortunately there are also some disadvantages for example it is
easy to get library out of sync between two versions of functions or that we
need extra define to compile program with casper support.
This isn't an ideal solution but it's good enough for now and should simplify
capsicumizing programs. This also doesn't close us any other ways to do those
mocks and this should evolve in time.

Discussed with: pjd, emaste, ed, rwatson, bapt, cem, bdrewery
Differential Revision: https://reviews.freebsd.org/D8753

Move libcasper tests from regression/capsicum/libcasper/ to
lib/libcasper/service/${service_name}/tests.

Reviewed by: emaste, ngie
Differential Revision: https://reviews.freebsd.org/D7759