client/agent: add clipboard max size limit for sending to/from the guest agent
client/agent: add clipboard max size limit for sending to/from the guest agent

define default & env variable for overriding it

vdiot/spice-protocol,Fantu/spice-protocol,SPICE/spice-protocol,Fantu/spice-protocol,freedesktop-unofficial-mirror/spice__spice-protocol,elmarco/spice-protocol,dezelin/spice-protocol,fgouget/spice-protocol,dezelin/spice-protocol,elmarco/spice-protocol,freedesktop-unofficial-mirror/spice__spice-protocol,SPICE/spice-protocol,vdiot/spice-protocol,SPICE/spice-protocol,dezelin/spice-protocol,fgouget/spice-protocol,vdiot/spice-protocol,elmarco/spice-protocol,fgouget/spice-protocol