Relax Java preproc_line_file unit test to pass in a separate build directory.

Exact paths comparison doesn't work when SWIG is built in a directory
different from the source one, so check whether the path just ends with the
expected path components instead.

This allows all Java tests to pass in this build configuration.

closes #115
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.
Don't accept strings too long to fit in char[N] with trailing NUL.

It was previously possible to assign "hello" to a variable backed by char[5]
storage in C, and the array contained the correct character data but without
the trailing NUL, which was unexpected in C.

This is not allowed any more, only "helo" can fit into a char[5] now and
anything else fails the type check, just as it already happened for the longer
strings before.

Closes #122