Implement signature as an array.

Store the length of the array in a variable. This makes the following rule
run (though not work):

rule authenticode_array_test {
  condition:
    for any i in (0..pe.signature_length):
      (pe.signature[i].issuer == "/CN=Fedora Secure Boot CA")
}

When run against shim-0.2-3.2.fc18.x86_64.efi it does not find the second
certificate. Still need to debug why.