With -count, if the first argument is a number, it is made available
with <count>. Problem is, there is always a default count it is impossible
to tell whether the user set it.
Since v:count and v:count1 still work with 'keywordprg', -count is
unnecessary. But 'keywordprg' still calls ':Man' with a count prefixed.
So it must still accept a count in the line number position, but not consume
the first argument. This is done with -range.
Fixes#5202.
3ff252ca86