Improved psql tab completion for joins

Hi,

Here is a set of small patches which improve the tab completion of joins
in psql. The completion of select queries and DML is very primitive in
psql but since we already have completion of relation names after FROM
and JOIN I think these small additions fit and at least I find them
useful myself. I don't use USING that much in application code but when
writing ad hoc queries I use it quite a bit.

## 0001-Complete-LATERAL-keyword-for-joins.patch

Adds support for the LATERAL keyword after JOIN.

## 0002-Complete-ON-and-USING-keywords-for-joins.patch

Adds completion of the ON and USING keywords.

## 0002-Complete-ON-and-USING-keywords-for-joins.patch

Adds completion of the first USING column.

Andreas