Attributes GH PR #979: https://github.com/ArcticaProject/nx-libs/pull/979

This used to be printed only in TEST mode. Some while ago I had
changed that to WARNING (because it is a warning...). However, this
happens e.g. when running the xscreensaver vfeedback module and it
does not look like it is a problem at all. So let's suppress this
warning again and leave it to the TEST mode as it used to be.

although there is no functional difference...

Attributes GH PR #978: https://github.com/ArcticaProject/nx-libs/pull/978

fullscreen= now accepts the new value "2" for a fullscreen covering
only one screen. -geometry and geometry= now know additional keywords
"allscreens" and "onescreen".

Fixes ArcticaProject/nx-libs#923

This reverts commit 4eba4f53.

Attributes GH PR #984: https://github.com/ArcticaProject/nx-libs/pull/984

just as the rest of the Xserver is alsow doing

nxagentWindowTopLevel() and nxagentNeedConnectionChange() return Boolean
nxagentPixmapIsVirtual() and nxagentIsShmPixmap(), too.