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/options.lua

@@ -1918,7 +1918,7 @@ return {
       vi_def=true,
       varname='p_scbk',
       redraw={'current_buffer'},
-      defaults={if_true={vi=-1}}
+      defaults={if_true={vi=1000}}
     },
     {
       full_name='scrollbind', abbreviation='scb',