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.

Lib/typemaps/wstring.swg

@@ -18,8 +18,20 @@ SWIG_pwchar_descriptor()
 }
 }
 
+%fragment("SWIG_wcsnlen","header",fragment="SWIG_FromWCharPtrAndSize") {
+size_t
+SWIG_wcsnlen(const wchar_t* s, size_t maxlen)
+{
+  const wchar_t *p;
+  for (p = s; maxlen-- && *p; p++)
+    ;
+  return p - s;
+}
+}
+
 %include <typemaps/strings.swg>
 %typemaps_string(%checkcode(UNISTRING), %checkcode(UNICHAR),
-		 wchar_t, WChar, SWIG_AsWCharPtrAndSize, SWIG_FromWCharPtrAndSize, wcslen,
+		 wchar_t, WChar, SWIG_AsWCharPtrAndSize, SWIG_FromWCharPtrAndSize,
+		 wcslen, SWIG_wcsnlen,
 		"<wchar.h>", WCHAR_MIN, WCHAR_MAX)