Use XORG_LOCK_DIR for Xorg lock files and sockets

The case of firefox 60 described in the commit messages showed that we should not use $TMPDIR instead of /tmp in our patch to isolate $DISPLAYs.

This introduce a new environment variable XORG_LOCK_DIR with clearer semantics.

Softwares relying on this have been updated or removed when not used.

The following softwares where already OK:

• ERP5 and erp5testnode did not set $TMPDIR and were using /tmp/
• cloudooo runs in --headless mode