fs: Add global flag '--color' to control terminal colors

* fs: add TerminalColorMode type
* fs: add new config(flags) for TerminalColorMode
* lib/terminal: use TerminalColorMode to determine how to handle colors
* Add documentation for '--terminal-color-mode'
* tree: remove obsolete --color replaced by global --color

This changes the default behaviour of tree. It now displays colors by
default instead of only displaying them when the flag -C/--color was
active. Old behaviour (no color) can be achieved by setting --color to
'never'.

Fixes: #6604

cmd/tree/tree.go

@@ -61,7 +61,6 @@ func init() {
 	flags.StringVarP(cmdFlags, &sort, "sort", "", "", "Select sort: name,version,size,mtime,ctime")
 	// Graphics
 	flags.BoolVarP(cmdFlags, &opts.NoIndent, "noindent", "", false, "Don't print indentation lines")
-	flags.BoolVarP(cmdFlags, &opts.Colorize, "color", "C", false, "Turn colorization on always")
 }
 
 var commandDefinition = &cobra.Command{
@@ -116,6 +115,7 @@ For a more interactive navigation of the remote see the
 		opts.SizeSort = sort == "size"
 		ci := fs.GetConfig(context.Background())
 		opts.UnitSize = ci.HumanReadable
+		opts.Colorize = ci.TerminalColorMode != fs.TerminalColorModeNever
 		if opts.DeepLevel == 0 {
 			opts.DeepLevel = ci.MaxDepth
 		}