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.