seccomp: Selftest for detection of filter flag support

Userspace needs to be able to reliably detect the support of a filter
flag. A good way of doing that is by attempting to enter filter mode,
with the flag bit(s) in question set, and a NULL pointer for the args
parameter of seccomp(2). EFAULT indicates that the flag is valid and
EINVAL indicates that the flag is invalid.

This patch adds a selftest that can be used to test this method of
detection in userspace.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>