Optional XPath operation limit

Optionally limit the maximum numbers of XPath operations when evaluating
an expression. Useful to avoid timeouts when fuzzing. The following
operations count towards the limit:

- XPath operations
- Location step iterations
- Union operations

Enabled by setting opLimit to a non-zero value. Note that it's the user's
responsibility to reset opCount. This allows to enforce the operation
limit across multiple reuses of an XPath context.

include/libxml/xpath.h

@@ -70,7 +70,8 @@ typedef enum {
     XPATH_INVALID_CHAR_ERROR,
     XPATH_INVALID_CTXT,
     XPATH_STACK_ERROR,
-    XPATH_FORBID_VARIABLE_ERROR
+    XPATH_FORBID_VARIABLE_ERROR,
+    XPATH_OP_LIMIT_EXCEEDED
 } xmlXPathError;
 
 /*
@@ -352,6 +353,10 @@ struct _xmlXPathContext {
 
     /* Cache for reusal of XPath objects */
     void *cache;
+
+    /* Resource limits */
+    unsigned long opLimit;
+    unsigned long opCount;
 };
 
 /*