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/test-suite/python/primitive_types_runme.py

@@ -165,7 +165,7 @@ t.var_paramc = sct_paramc
 t.v_check()
 
 # this value contains a '0' char!
-if def_namet != 'ho\0la':
+if def_namet != 'hola':
   print "bad namet", def_namet
   raise RuntimeError