selftests/seccomp: Make kcmp() less required

The seccomp tests are a bit noisy without CONFIG_CHECKPOINT_RESTORE (due
to missing the kcmp() syscall). The seccomp tests are more accurate with
kcmp(), but it's not strictly required. Refactor the tests to use
alternatives (comparing fd numbers), and provide a central test for
kcmp() so there is a single SKIP instead of many. Continue to produce
warnings for the other tests, though.

Additionally adds some more bad flag EINVAL tests to the addfd selftest.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@chromium.org>
Cc: linux-kselftest@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: bpf@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>