patch 8.0.1712: terminal scrollback is not limited

Problem:    Terminal scrollback is not limited.
Solution:   Add the 'terminalscroll' option.

src/option.c

@@ -2748,6 +2748,15 @@ static struct vimoption options[] =
 #else
 			    (char_u*)NULL, PV_NONE,
 			    {(char_u *)FALSE, (char_u *)FALSE}
+#endif
+			    SCRIPTID_INIT},
+    {"terminalscroll", "tlsl", P_NUM|P_VI_DEF|P_VIM|P_RBUF,
+#ifdef FEAT_TERMINAL
+			    (char_u *)&p_tlsl, PV_NONE,
+			    {(char_u *)10000L, (char_u *)10000L}
+#else
+			    (char_u *)NULL, PV_NONE,
+			    {(char_u *)NULL, (char_u *)0L}
 #endif
 			    SCRIPTID_INIT},
     {"termkey", "tk",	    P_STRING|P_ALLOCED|P_RWIN|P_VI_DEF,