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/perl5/primitive_types_runme.pl

@@ -166,7 +166,7 @@ $t->{var_paramd} = $primitive_types::sct_paramd;
 $t->{var_paramc} = $primitive_types::sct_paramc;
 ok($t->v_check(), 'v_check');
 
-is($primitive_types::def_namet, "ho\0la", "namet");
+is($primitive_types::def_namet, "hola", "namet");
 $t->{var_namet} = $primitive_types::def_namet;
 is($t->{var_namet}, $primitive_types::def_namet, "namet");