xutil: Implement xstrndup without relying on strndup.

xutil: Implement xstrndup without relying on strndup.

Since we need to do this for portability, (some systems don't have a
strndup function), we might as well do it unconditionally. There's
almost no disadvantage to doing so, and this has the advantages of not
requiring a configure-time check nor having two different
implementations, one of which would often be less tested.