Change the length of strings created from fixed-size buffers.

Use the usual C rule for NUL-terminated strings instead of discarding all the
trailing NUL characters.

This was unexpected (as buffers in C code are not necessarily always padded
with NULs to their full length) and also inconsistent among languages as this
was only done for those of them using typemaps/strings.swg but not for C# or
Java, for example, which terminated the string at the first NUL even before
this change.

Notice that this patch couldn't use strlen() or wcslen() with possibly not
NUL-terminated strings, so we had to add [our own equivalents of] strnlen()
and wcsnlen() and use them instead. This required adding yet another parameter
to string typemap macros, so update the example using them accordingly too.

Examples/perl5/xmlstring/xmlstring.i

@@ -93,6 +93,17 @@ SWIG_FromXMLChPtrAndSize(const XMLCh* input, size_t size)
 }
 }
 
+%fragment("SWIG_XMLStringNLen","header") {
+size_t
+SWIG_XMLStringNLen(const XMLCh* s, size_t maxlen)
+{
+  const XMLCh *p;
+  for (p = s; maxlen-- && *p; p++)
+    ;
+  return p - s;
+}
+}
+
 %init {
   if (!SWIG_UTF8Transcoder()) {
     croak("ERROR: XML::Xerces: INIT: Could not create UTF-8 transcoder");
@@ -106,6 +117,7 @@ SWIG_FromXMLChPtrAndSize(const XMLCh* input, size_t size)
 		 SWIG_AsXMLChPtrAndSize, 
 		 SWIG_FromXMLChPtrAndSize,
 		 XERCES_CPP_NAMESPACE::XMLString::stringLen,
+		 SWIG_XMLStringNLen,
 		 "<XMLCh.h>", INT_MIN, INT_MAX);