Add pane-border-lines value to use spaces for pane borders, from lukas

dot 23022005 at gmail dot com in GitHub issue 4587.

tmux.1

@@ -5078,6 +5078,8 @@ heavy lines using UTF-8 characters
 simple ASCII characters
 .It number
 the pane number
+.It spaces
+space characters
 .El
 .Pp
 .Ql double