* lisp/progmodes/sql.el (sql-postgres-list-databases): Handle non-zero
exit statuses from `psql -ltX' more gracefully by returning nil.
* test/lisp/progmodes/sql-tests.el
(sql-tests-postgres-list-databases-error): New test.
* lisp/progmodes/sql.el (sql-postgres-login-params): Complete database
name.
(sql-postgres-list-databases): New function returning a list of
available PostgreSQL databases.
* test/lisp/progmodes/sql-tests.el: New file with tests for sql.el.