#512650 by sdboyer: Multiple many to one arguments on the same relationship were not relationship safe.

Fix accidentally reverted changes. Plus get rid of an ugly piece of hardcoding around the signature.

#319167: add_table() would return TRUE if the table was already added but the expected return is the alias.

query::add_field() called ensure_table too aggressively and could cause aliases to improperly overwrite already existing tables.

#282864: Fix collapsing relationships in CCK fields that was returning wrong value and breaking collapse.

query::add_relationship was adjusting the join for the relationship prematurely; add_table will do that on its own.

Fix a problem with relationships that caused the adjust_join to sometimes not be run, thanks to a minor error in the join collapsing code.