seccomp: prefer SCMP_ACT_KILL_PROCESS if available

The upcoming libseccomp release should have SCMP_ACT_KILL_PROCESS
action (https://github.com/seccomp/libseccomp/issues/96

).

SCMP_ACT_KILL_PROCESS is preferable to immediately terminate the
offending process, rather than having the SIGSYS handler running.

Use SECCOMP_GET_ACTION_AVAIL to check availability of kernel support,
as libseccomp will fallback on SCMP_ACT_KILL otherwise, and we still
prefer SCMP_ACT_TRAP.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Eduardo Otubo <otubo@redhat.com>