nginx: Adds proper support to SecServerSignature

SecServerSignature was leading nginx to crash. It was trying to write over a
memory area that it was not allowed to. In order to fix that a new function was
created on the standalone api. This function is called
modsecIsServerSignatureAvailale. Whenever it returns data it means that the
function SecServerSignature was used by the user. Nginx module was also patched
to support this new function.