Looks like /etc/X11/Xsession is not executed, but the files in /etc/X11/Xsession.d/ are sourced somehow. My best guess is that there's some other mechanism emulating Xsession's behaviour but without defining has_option.
Maybe this is a pending bug introduced in an earlier version of Ubuntu that has come to light just now, after has_option has been moved as #3 points out.
Looks like /etc/X11/Xsession is not executed, but the files in /etc/X11/ Xsession. d/ are sourced somehow. My best guess is that there's some other mechanism emulating Xsession's behaviour but without defining has_option.
Maybe this is a pending bug introduced in an earlier version of Ubuntu that has come to light just now, after has_option has been moved as #3 points out.
BTW: I'm using lightdm with the Arctica Greeter.