treat "not present" in preferred content as invalid

treat "not present" in preferred content as invalid

Detect when a preferred content expression contains "not present", which
would lead to repeatedly getting and then dropping files, and make it never
match. This also applies to "not balanced" and "not sizebalanced".

--explain will tell the user when this happens

Note that getMatcher calls matchMrun' and does not check for unstable
negated limits. While there is no --present anyway, if there was,
it would not make sense for --not --present to complain about
instability and fail to match.