unix: move clipboard utility discovery out of init()

[tstromberg]

Fixes #63

This moves the clipboard utility code from init() to a function named findClipboardUtility(). While this has a minor performance impact if the WriteAll or ReadAll command is called in a loop, this does improve binary start time.

It also looks a whole lot less scary in system call traces. This is particularly important for programs that indirectly import but never use this library.

As part of the cleanup, I removed references to an unused Primary global. Please let me know if you would like me to put them back.

[norbjd]

Hi 👋 just a small comment: instead of instantiating a new commandInfo every time readAll() or writeAll() is called, should it be possible to create a singleton:

 var instance commandInfo
 var once sync.Once

 func GetInstance() commandInfo {
     once.Do(func() {
         instance = findClipboardUtility()
     })
     return instance
 }

And call c := GetInstance() instead of c := findClipboardUtility()?

It will be quicker since findClipboardUtility() will be called only once (at the first call).

What do you think? BTW, thanks for this PR 🙌

[tstromberg]

I considered this option, but it didn't seem worth the additional complication for a call that is unlikely to be called very often. If making this change helps the PR get merged though, I'm all for it.