This is to give a home to strtok_len. It's a bit silly to add a header
for one routine, but it needs to be shared between several compilation
units (or at least that's the most natural design).
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.
