clipboard: avoid clipboard during :global. #2809

This is equivalent to patches 7.4.396, 7.4.445 and 7.4.598.

vim-patch:7.4.396

Problem:    When 'clipboard' is "unnamed", :g/pat/d is very slow.
(Praful)
Solution:   Only set the clipboard after the last delete. (Christian
Brabandt)

1f285eb49a

vim-patch:7.4.445

Problem:    Clipboard may be cleared on startup.
Solution:   Set clip_did_set_selection to -1 during startup. (Christian
	    Brabandt)

1a19d37d90

vim-patch:7.4.598

Problem:    ":tabdo windo echo 'hi'" causes "* register not to be
changed.
	    (Salman Halim)
Solution:   Change how clip_did_set_selection is used and add
	    clipboard_needs_update and global_change_count.  (Christian
	    Brabandt)

af6a579263

Co-Author: @bfredl

src/nvim/globals.h

@@ -519,8 +519,6 @@ EXTERN int sys_menu INIT(= FALSE);
  * ('lines' and 'rows') must not be changed. */
 EXTERN int updating_screen INIT(= FALSE);
 
-
-
 /*
  * All windows are linked in a list. firstwin points to the first entry,
  * lastwin to the last entry (can be the same as firstwin) and curwin to the