Deprecate and replace the "tabset" command

Deprecate and replace the "tabset" command

The "tabset" command sets the tab width as used by the TUI for source
and disassembly display.

This command has long seemed to be misnamed to me.  It is more in
keeping with gdb design to call it "set tui tab-width".  Also, making
this change allows for the corresponding "show" command to work.

gdb/ChangeLog
2018-10-19  Tom Tromey  <tom@tromey.com>

	PR tui/18388:
	* NEWS: Mention tabset deprecation.
	* tui/tui-win.c (tui_tab_width, internal_tab_width): New globals.
	(update_tab_width): New function.
	(tui_set_tab_width, tui_show_tab_width): New functions.
	(tui_set_tab_width_command): Use update_tab_width.
	(_initialize_tui_win): Move to end of file.  Deprecate "tabset".
	Add new "set tui tab-width" command.
	* tui/tui-source.c (tui_set_source_content): Update.
	* tui/tui-disasm.c (tui_set_disassem_content): Update.
	* tui/tui-data.h (tui_default_tab_len, tui_set_default_tab_len):
	Don't declare.
	(tui_tab_width): Declare.
	* tui/tui-data.c (default_tab_len, tui_default_tab_len)
	(tui_set_default_tab_len): Remove.

gdb/doc/ChangeLog
2018-10-19  Tom Tromey  <tom@tromey.com>

	PR tui/18388:
	* gdb.texinfo (TUI Commands): Remove tabset documentation.
	(TUI Configuration): Document "set tui tab-width".