Better clipboard program defaults and warnings

[docwhat]

The current system of fallbacks in brittle and is tricky; since different versions of some tools have different pros and cons.

In addition (as pointed out in #72) there are some places tmux-yank can do a better reasonable default.

Especially with tmux's set-clipboard (only on Linux xterm and macOS with iTerm2) can get rid of the need for any clipboard program!

Goal:

• Rework the current system to have better defaults where possible.
• Use set-clipboard if the user says it is set up correctly.
• Allow the user better control of which clipboard program(s) to use.