Restore the former RestrictInfo field valid_everywhere (but invert the flag

sense and rename to "outerjoin_delayed" to more clearly reflect what it
means).  I had decided that it was redundant in 8.1, but the folly of this
is exposed by a bug report from Sebastian Böck.  The place where it's
needed is to prevent orindxpath.c from cherry-picking arms of an outer-join
OR clause to form a relation restriction that isn't actually legal to push
down to the relation scan level.  There may be some legal cases that this
forbids optimizing, but we'd need much closer analysis to determine it.