patch 8.0.1592: terminal windows in a session are not properly restored

Problem:    Terminal windows in a session are not properly restored.
Solution:   Add "terminal" in 'sessionoptions'.  When possible restore the
            command running in a terminal.
diff --git a/src/option.c b/src/option.c
index 3a0a659..084e5e1 100644
--- a/src/option.c
+++ b/src/option.c
@@ -2403,7 +2403,7 @@
     {"sessionoptions", "ssop", P_STRING|P_VI_DEF|P_ONECOMMA|P_NODUP,
 #ifdef FEAT_SESSION
 			    (char_u *)&p_ssop, PV_NONE,
-	 {(char_u *)"blank,buffers,curdir,folds,help,options,tabpages,winsize",
+	 {(char_u *)"blank,buffers,curdir,folds,help,options,tabpages,winsize,terminal",
 							       (char_u *)0L}
 #else
 			    (char_u *)NULL, PV_NONE,