UPSTREAM: hardening: Clarify Kconfig text for auto-var-init

Clarify the details around the automatic variable initialization modes
available. Specifically this details the values used for pattern init
and expands on the rationale for zero init safety. Additionally makes
zero init the default when available.

Cc: glider@google.com
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: linux-security-module@vger.kernel.org
Cc: clang-built-linux@googlegroups.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
(cherry picked from commit dcb7c0b9461c2a30f6616262736daac6f01ecb09)
Change-Id: Id2a3b2b2953677e29e6deb931350b04091474b08
Signed-off-by: Nathan Chancellor <nathan@kernel.org>