client: rename X2GO-specific display setting script to have a generic name

Display settings can be defined in multiple locations:

- Puavo
- $HOME/.config/monitors.xml
- $HOME/.x2go/C-X2GO_SESSION/options

To handle all these settings properly, display setup must be
centralized. Hence the script is renamed and it will be *the place* for
all display setup spells.
client: set -eu instead of set -e in puavo-display-setup
client: apply Puavo-defined display settings in user session

In Quantal, desktop session seemed to "inherit" display settings from
its parent, lightdm. In Trusty, that is not the case and hence we need
to apply them here. Puavo-defined settings are applied only if the user
has not made any local settings.