Yama: add additional ptrace scopes

This expands the available Yama ptrace restrictions to include two more
modes. Mode 2 requires CAP_SYS_PTRACE for PTRACE_ATTACH, and mode 3
completely disables PTRACE_ATTACH (and locks the sysctl).

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>