terminal: global 'scrollback' #6352

Make the 'scrollback' option work like most other buffer-local options:
- `:set scrollback=x` sets the global and local value
- `:setglobal scrollback=x` sets only the global default
- new terminal buffers inherit the global

Normal buffers are still always -1, and :setlocal there is an error.

Closes #6337

src/nvim/globals.h

@@ -1255,7 +1255,7 @@ typedef enum {
   kCdScopeInvalid = -1,
   kCdScopeWindow,  ///< Affects one window.
   kCdScopeTab,     ///< Affects one tab page.
-  kCdScopeGlobal,  ///< Affects the entire instance of Neovim.
+  kCdScopeGlobal,  ///< Affects the entire Nvim instance.
 } CdScope;
 
 #define MIN_CD_SCOPE  kCdScopeWindow