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/option_defs.h

@@ -524,6 +524,7 @@ EXTERN int p_ru;                // 'ruler'
 EXTERN char_u   *p_ruf;         // 'rulerformat'
 EXTERN char_u   *p_pp;          // 'packpath'
 EXTERN char_u   *p_rtp;         // 'runtimepath'
+EXTERN long p_scbk;             // 'scrollback'
 EXTERN long p_sj;               // 'scrolljump'
 EXTERN long p_so;               // 'scrolloff'
 EXTERN char_u   *p_sbo;         // 'scrollopt'
@@ -811,4 +812,6 @@ enum {
 /* Value for b_p_ul indicating the global value must be used. */
 #define NO_LOCAL_UNDOLEVEL -123456
 
+#define SB_MAX 100000  // Maximum 'scrollback' value.
+
 #endif // NVIM_OPTION_DEFS_H