* clipboard: Reduce unnecessary special-casing on stdin

Ideally the parameter would just be removed-users could always
just do "clipcopy < some-file". but removing the parameter would break
backwards compatibility.

In any case, this simplifies the logic considerably.

* clipboard: Avoid unnecessary re-detection each time

Previously, OS detection would happen on each invocation. This makes it
happen once (unless it fails, in which case it will try again on the
next invocation).

This has the additional benefit of localizing the platform-specific
checks and commands, too, versus spreading them out in separate
functions.

* clipboard: Add support for several more clipboards

This implements essentially the same heuristic as neovim, with the additional
(existing) special support for Cygwin.
See: https://github.com/neovim/neovim/blob/e682d799fa3cf2e80a02d00c6ea874599d58f0e7/runtime/autoload/provider/clipboard.vim#L55-L121
  - pbcopy, pbpaste (macOS)
  - cygwin (Windows running Cygwin)
  - wl-copy, wl-paste (if $WAYLAND_DISPLAY is set)
  - xclip (if $DISPLAY is set)
  - xsel (if $DISPLAY is set)
  - lemonade (for SSH) https://github.com/pocke/lemonade
  - doitclient (for SSH) http://www.chiark.greenend.org.uk/~sgtatham/doit/
  - win32yank (Windows)
  - tmux (if $TMUX is set)

* clipboard: Fix tmux clipcopy after testing

Tmux must have special handling for /dev/stdin since it's managing the
terminal itself. This was tested with tmux-2.9a on macOS.

* clipboard: Fix bad expansion of exit-code test