I personally find it more convenient to have pasted data written to
the X11 PRIMARY selection, so that I can paste it with a fast middle-
button click, than to write to CLIPBOARD which typically needs a key
sequence or menu action.
This commit adds a command-line option to let me express that
preference: now I can say "/clipboard:use-selection:PRIMARY" on the
command line, which not only enables clipboard transfer but also says
which X selection I want it to talk to. The previous options
"+clipboard" and "-clipboard" are also still supported.
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