Search LIST for a valid buffer to display in FRAME.
Return nil when all buffers in LIST are undesirable for display,
otherwise return the first suitable buffer in LIST.
Buffers not visible in windows are preferred to visible buffers,
unless VISIBLE-OK is non-nil.
If the optional argument FRAME is nil, it defaults to the selected frame.
If BUFFER is non-nil, ignore occurrences of that buffer in LIST.