zone.lisp: Split a single `:txt' string into small enough pieces.

The substrings of a `:txt' record can be at most 255 bytes long.  If the
argument is a single string that's too long then split it into pieces;
prefer to split at semicolons, or spaces.

If the argument is a list of strings, then respect their split.
Theoretically, the split positions are semantically transparent, but
it's possible that some programs are sensitive to the boundaries.