parser: Introduce XML_INPUT_NETWORK input flag

This allows to disable network access when creating parser inputs with
xmlInputCreateUrl.

include/libxml/parserInternals.h

@@ -363,6 +363,7 @@ XMLPUBFUN xmlParserInputPtr
 #define XML_INPUT_BUF_STATIC		(1u << 1)
 #define XML_INPUT_BUF_ZERO_TERMINATED	(1u << 2)
 #define XML_INPUT_UNZIP                 (1u << 3)
+#define XML_INPUT_NETWORK               (1u << 4)
 
 XMLPUBFUN int
 xmlInputCreateUrl(const char *url, int flags, xmlParserInputPtr *out);