worker: get PATH from parent instead of #define

If no PATH is set, then the session worker tries to set one up,
based on guess. This commit changes GDM to just use the PATH given
to GDM itself, rather than guessing (and getting it wrong).

https://bugzilla.gnome.org/show_bug.cgi?id=727980
session: introduce new client-rejected signal

If a client gets rejected because it's not allowed to connect
to a particular session, we really need to inform the owner
of the session object so it can do any clean up it needs to do,
if necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=727982
manager: don't leak session objects when client is rejected

if a client creates a transient reauthentication session and then we reject
the client when it tries to connect to it, then we need to clean up
the associated session object.

This commit does that.

https://bugzilla.gnome.org/show_bug.cgi?id=727982
manager: be more accepting of callers with uids different from their session

If a user runs su in their session, that switched user will now be
running in a session that doesn't belong to it.  GDM won't allow a user
access to the worker process associated with the session in this case.
Instead, it will try to create a temporary just-in-time reauthentication
channel so reauthentication can happen without having the user talking to
another user's worker. Unfortunately, a logic error in the code means,
the user won't access to its own just-in-time channel.

This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=727982
session: don't set DISPLAY for sessions that bring their own

We only want to set DISPLAY if the user session is sharing the display
with the login screen.

https://bugzilla.gnome.org/show_bug.cgi?id=728204
session: drop remnants of WINDOWPATH

https://bugzilla.gnome.org/show_bug.cgi?id=728204
session: make sure session display mode is set even if session isn't explicitly picked

Right now we we only set the display mode when a user picks a session
from the session list.  That means the wrong display mode will get used
if wayland is saved from a previous log in.

This commit makes sure the display mode gets set as soon as the user
credentials are established, since at the point log in is imminent.

https://bugzilla.gnome.org/show_bug.cgi?id=728204
Updated Spanish translation