Using arguments that need a parameter without providing one would lead
a segfault due to calling strcmp() will NULL. Triggered by running
"nxagent -ac :1 -options nx/nx,fullscreen" or similar,

Fixes ArcticaProject/nx-libs#847

Attributes GH PR #846: https://github.com/ArcticaProject/nx-libs/pull/846

Use a more descriptive name.

Fixes ArcticaProject/nx-libs#632

This avoids delays, timeouts and missing copy/paste actions.

add new debug output, extend existing, drop meaningless

serverClientCutProperty is the external equivalent of the internal
clientCutProperty. We need it on the server side, too, because we use
the property on the serverWindow on the real X server. We could
(mis)use serverCutProperty here but this might introduce race
conditions when both sides request selections simultaneously.

This is not a functional change, it only helps in clearer debugging
output.

The second parameter was always NULL, so remove it. Also remove some
debugging output which distracts while debugging as the function is
called from ProcChangeProperty for _every_ property, also non-clipboard
related ones.

we reference lastSelectionOwner[nxagentClipboardSelection].selection everywhere.

nxagentClipboardAtom -> serverCLIPBOARD
nxagentTimestampAtom -> serverTIMESTAMP

This is to match the names of the other server atom variables.

At some places we were using NumCurrentSelections. We replace that by
nxagentMaxSelections because they always have the identical value.

Just as everyone else does. ICCCM is not really clear about this.

endTransfer is the only caller

fixup! Clipboard.c: introduce XSendNotificationEventHelper

There was only one place where this was checked but we check it always.

Attributes GH PR #827: https://github.com/ArcticaProject/nx-libs/pull/827