pam_systemd: complement per-area $HOME management with per-area $XDG_RUNTIME_DIRECTOR...

pam_systemd: complement per-area $HOME management with per-area $XDG_RUNTIME_DIRECTORY mgmt

When a user logs into a non-default area we give them a private
$HOME for that area (that's what 'area' is supposed to be after all). We
so far left $XDG_RUNTIME_DIRECTORY as it was. Let's change that and
mirror the subdirectory logic there too.

Why? $XDG_RUNTIME_DIR is generally the place where AF_UNIX sockets are
bound that can be used to connect to per-user services. (in particular
all those which are behind D-Bus.) If we don't patch $XDG_RUNTIME_DIR
like this then this means all the backing services will use the main
area, which is problematic (since clients and services will disagree on
$HOME), and makes it impossible to support the area concept for
graphical logins properly.

This does not actually make graphical logins work, but it at least makes
them fail cleanly. That's because this patch alone won't make sure a
per-area service manager/dbus instance is invoked automatically. That
however can be added later, in a patch to logind.