Pass X server name in parameters.

Hence we can use xorg-launch-helper with servers like Xvnc.
Change units' concept: make units templated, get rid of xorg.target.

- create unit xorg@.service which launches Xorg at display %I
and, when installed, aliases itself to xserver@.service

- create helper units need-xorg@.service and need-xorg@.target
which encode (better) dependency on an X server using
'Requires=xserver@.service' and 'After=xserver@.service'

- get rid of xorg.target because it is no longer needed
Shed some history.
Rename units: need-xorg@ -> need-xserver@.

The latter name (xserver) shows their purpose better: require
any X server, not only Xorg.
Fix -xorg -> -xserver renaming: alter build system accordingly.
Add appropriate automake option