Allow remote desktop sharing to work again on Linux

[doublec]

Git commit 882d8cf2 fixed a problem where restarting a snapshot that had open worlds would fail on Linux due to using the fully qualified display name. Unfortunately it broke support for sharing worlds on other displays - they would always open on the local display.

The fix backs out that change so that remote displays work and fixes the original issue by using the default display name.

This can be tested by starting an additional Xserver like Xnest:

$ Xnest -ac :1 &

Then from within Self on the local display grab a 'Share window with' Morph from the factory onto the desktop. Enter '127.0.0.1:1.0' and press 'share window with'. The Xnest will now show a Self desktop and you can see the other user.

Prior to this patch the desktop would always spawn in a new window on the local display regardless of the display name used.

Saving the snapshot, exiting, then restarting correctly opens the windows on the local display.

[doublec]

Rebased on top of master.