Handle unix_socket_directories GUC change in PostgreSQL 9.3

PostgreSQL replaced unix_socket_directory GUC with
unix_socket_directories.  Handle that in places where this GUC is used.
test: Fix SchemaNameError test

PostgreSQL 9.3 raises UndefinedTableError instead of SchemaNameError
when a reference to a non-existent table in a non-existent schema is
made.  Fix the test to test for non-existent schema specifically.
test: Fix MessageHook test on PostgreSQL 9.3

PostgreSQL 9.3 no longer raises a NOTICE about automatic creation of an
implicit index when PRIMARY KEY is declared (it raises a DEBUG1
instead).  Switch the test to an attempt to REINDEX a table without
indexes, which is a more consistent NOTICE (this has been a WARNING
before PostgreSQL 7.4).