defaults: set 'mouse' to 'a' by default. #2860

Re: https://github.com/neovim/neovim/issues/2676

Also, some documentation changes.

runtime/doc/help.txt

@@ -8,8 +8,7 @@ Close this window:  Use ":q<Enter>".
    Get out of Vim:  Use ":qa!<Enter>" (careful, all changes are lost!).
 
 Jump to a subject:  Position the cursor on a tag (e.g. |bars|) and hit CTRL-].
-   With the mouse:  ":set mouse=a" to enable the mouse (in xterm or GUI).
+   With the mouse:  Double-click the left mouse button on a tag, e.g. |bars|.
-		    Double-click the left mouse button on a tag, e.g. |bars|.
 	Jump back:  Type CTRL-T or CTRL-O (repeat to go further back).
 
 Get specific help:  It is possible to go directly to whatever you want help

runtime/doc/options.txt

@@ -4528,7 +4528,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	listing continues until finished.
 
 						*'mouse'* *E538*
-'mouse'			string	(default "", "a" for GUI, MS-DOS and Win32)
+'mouse'			string	(default "a")
 			global
 
 	Enable the use of the mouse.  Only works for certain terminals.

runtime/doc/term.txt

@@ -522,10 +522,6 @@ This section is about using the mouse on a terminal or a terminal window.  How
 to use the mouse in a GUI window is explained in |gui-mouse|.  For scrolling
 with a mouse wheel see |scroll-mouse-wheel|.
 
-Don't forget to enable the mouse with this command: >
-	:set mouse=a
-Otherwise Vim won't recognize the mouse in all modes (See 'mouse').
-
 These characters in the 'mouse' option tell in which situations the mouse will
 be used by Vim:
 		n	Normal mode
@@ -536,10 +532,6 @@ be used by Vim:
 		a	all previous modes
 		r	for |hit-enter| prompt
 
-The default for 'mouse' is empty, the mouse is not used.  Normally you would
-do: >
-	:set mouse=a
-to start using the mouse (this is equivalent to setting 'mouse' to "nvich").
 If you only want to use the mouse in a few modes or also want to use it for
 the two questions you will have to concatenate the letters for those modes.
 For example: >
@@ -559,10 +551,8 @@ pressed go to the xterm.  With the currently active mode not included in
 'mouse' all mouse clicks go to the xterm.
 
 							*xterm-clipboard*
-In the Athena and Motif GUI versions, when running in a terminal and there is
+The middle mouse button will insert the unnamed register.  In that case, here
-access to the X-server (DISPLAY is set), the copy and paste will behave like
+is how you copy and paste a piece of text:
-in the GUI.  If not, the middle mouse button will insert the unnamed register.
-In that case, here is how you copy and paste a piece of text:
 
 Copy/paste with the mouse and Visual mode ('mouse' option must be set, see
 above):
@@ -577,10 +567,6 @@ Shortcut: If the insert position is on the screen at the same time as the
 Visual text, you can do 2, 3 and 4 all in one: Click the middle mouse button
 at the insert position.
 
-							*xterm-command-server*
-When the X-server clipboard is available, the command server described in
-|x11-clientserver| can be enabled with the --servername command line argument.
-
 							*xterm-copy-paste*
 NOTE: In some (older) xterms, it's not possible to move the cursor past column
 95 or 223.  This is an xterm problem, not Vim's.  Get a newer xterm

runtime/doc/vim_diff.txt

@@ -33,6 +33,7 @@ these differences.
 - 'backspace' defaults to "indent,eol,start"
 - 'encoding' defaults to "utf-8"
 - 'formatoptions' defaults to "tcqj"
+- 'mouse' defaults to "a"
 - 'nocompatible' is always set
 - 'nrformats' defaults to "hex"
 - 'smarttab' is set by default

src/nvim/option.c

@@ -1111,11 +1111,11 @@ static vimoption_T
   {"more",        NULL,   P_BOOL|P_VIM,
    (char_u *)&p_more, PV_NONE,
    {(char_u *)FALSE, (char_u *)TRUE} SCRIPTID_INIT},
-  {"mouse",       NULL,   P_STRING|P_VI_DEF|P_FLAGLIST,
+  {"mouse",       NULL,   P_STRING|P_FLAGLIST,
    (char_u *)&p_mouse, PV_NONE,
    {
      (char_u *)"",
-     (char_u *)0L
+     (char_u *)"a"
    } SCRIPTID_INIT},
   {"mousefocus",   "mousef", P_BOOL|P_VI_DEF,
    (char_u *)NULL, PV_NONE,