We keep the lib/xutil.c version. As a consequence, also factor out
_internal_error and associated macros. It might be overkill to make a
new file error_util.c for this, but _internal_error does not really
belong in database.cc.
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.
