Command-line option to choose an X selection.

Command-line option to choose an X selection.

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.

(cherry picked from commit 64948b96c4c4640078ce563b165a907251fd6f20)