Tweak joinlist creation to avoid generating useless one-element subproblems

when collapsing of JOIN trees is stopped by join_collapse_limit.  For instance
a list of 11 LEFT JOINs with limit 8 now produces something like
	((1 2 3 4 5 6 7 8) 9 10 11 12)
instead of
	(((1 2 3 4 5 6 7 8) (9)) 10 11 12)
The latter structure is really only required for a FULL JOIN.
Noted while studying an example from Shane Ambler.