seccomp: Rename SECCOMP_RET_KILL to SECCOMP_RET_KILL_THREAD

In preparation for adding SECCOMP_RET_KILL_PROCESS, rename SECCOMP_RET_KILL
to the more accurate SECCOMP_RET_KILL_THREAD.

The existing selftest values are intentionally left as SECCOMP_RET_KILL
just to be sure we're exercising the alias.

Signed-off-by: Kees Cook <keescook@chromium.org>