Use version-independent way to pass socket dir to postgres
    
    With version <9.3 we would need to specify unix_socket_directory
    and with >=9.3 we would need unix_socket_directories.
    Both versions have a -k flag which is equivalient to setting either
    option, so this makes it compatible across all versions.
    
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Pierre-Yves Chibon <pingou@pingoured.fr>
    Reviewed-by: Howard Johnson <merlin@merlinthp.org>