Emulate empty join conditions where needed.

Appending the dummy condition "on 1=1" works almost everywhere, except
in Access, where the condition has to reference columns from both sides
of the join, so we have to thread the dummy "1" values through the
sub-queries.

Fixes Typesafe support ticket 3140.
Test in JoinTest.testNoJoinCondition.
Merge pull request #700 from slick/tmp/support-3140

Emulate empty join conditions where needed.