pefile: Parse a PE binary to find a key and a signature contained therein

Parse a PE binary to find a key and a signature contained therein.  Later
patches will check the signature and add the key if the signature checks out.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>