signatures: Add data_end_offset to signature_match() and custom events

This change tracks the current offset (number of bytes fed into matchers)
on the top-level RuleEndpointState such that we can compute the relative ending
for matched texts individually.

Additionally, it adds the data_end_offset as a new optional parameter to
signature_match().

src/RuleAction.h

@@ -43,6 +43,7 @@ public:
 private:
     StringValPtr msg;
     EventHandlerPtr handler;
+    bool want_end_of_match; // Whether handler accepts end_of_match parameter.
 };
 
 class RuleActionMIME : public RuleAction {