This fix is entirely sick, so is our method of serializing units,
which we write out as (unit foo bar) instead of (unit "foo bar"). The
assumption that caused this shit was that a unit's "identifier" is
really an identifier in the C-ish sense, when in fact it's just a
random user entered string.

Here, we try to parse at least the default units shipped with gimp,
and we add code to parse (unit "foo bar") in order to be compatible
with future correct unit serializing.