AUTOCOMMIT mode is now an available backend GUC variable; setting it

AUTOCOMMIT mode is now an available backend GUC variable; setting it
to false provides more SQL-spec-compliant behavior than we had before.
I am not sure that setting it false is actually a good idea yet; there
is a lot of client-side code that will probably be broken by turning
autocommit off.  But it's a start.

Loosely based on a patch by David Van Wie.