Implement help/show values for 'set|show style'.

Implement help/show values for 'set|show style'.

Currently, the behaviour is:
  (gdb) show style
  (gdb) set style
  (gdb) show style address
  (gdb) set style address
  (gdb)

With this patch, the behaviour is:
  (gdb) show style
  style address background:  The "address" background color is: none
  style address foreground:  The "address" foreground color is: blue
  style address intensity:  The "address" display intensity is: normal
  enabled:  CLI output styling is enabled.
  style filename background:  The "filename" background color is: none
  style filename foreground:  The "filename" foreground color is: green
  style filename intensity:  The "filename" display intensity is: normal
  style function background:  The "function" background color is: none
  style function foreground:  The "function" foreground color is: yellow
  style function intensity:  The "function" display intensity is: normal
  style variable background:  The "variable" background color is: none
  style variable foreground:  The "variable" foreground color is: cyan
  style variable intensity:  The "variable" display intensity is: normal
  (gdb) set style
  "set style" must be followed by an appropriate subcommand.
  List of set style subcommands:

  set style address -- Address display styling
  set style enabled -- Set whether CLI styling is enabled
  set style filename -- Filename display styling
  set style function -- Function name display styling
  set style variable -- Variable name display styling

  Type "help set style" followed by set style subcommand name for full documentation.
  Type "apropos word" to search for commands related to "word".
  Command name abbreviations are allowed if unambiguous.
  (gdb) show style address
  background:  The "filename" background color is: none
  foreground:  The "filename" foreground color is: green
  intensity:  The "filename" display intensity is: normal
  (gdb) set style address
  List of set style address subcommands:

  set style address background -- Set the background color for this property
  set style address foreground -- Set the foreground color for this property
  set style address intensity -- Set the display intensity color for this property

  Type "help set style address" followed by set style address subcommand name for full documentation.
  Type "apropos word" to search for commands related to "word".
  Command name abbreviations are allowed if unambiguous.
  (gdb)

gdb/ChangeLog
	* cli/cli-style.h (class cli_style_option): <add_setshow_commands>
	Remove arg prefixname, add do_set and do_show.
	Add member functions set_list and show_list.
	* cli/cli-style.c (class cli_style_option): Update accordingly.
	(style_set_list): Move to file scope.
	(style_show_list): Likewise.
	(set_style): Call help_list.
	(show_style): Call cmd_show_list.
	(_initialize_cli_style): New macro STYLE_ADD_SETSHOW_COMMANDS.
	Update to use the new macro.