patch 8.0.0898: can't use the alternate screen in a terminal window

Problem:    Can't use the alternate screen in a terminal window.
Solution:   Initialze the alternate screen. (Yasuhiro Matsumoto, closes
            #1957)  Add term_getaltscreen().
diff --git a/src/evalfunc.c b/src/evalfunc.c
index b5880c5..6cb0ed7 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -831,6 +831,7 @@
 #endif
     {"tempname",	0, 0, f_tempname},
 #ifdef FEAT_TERMINAL
+    {"term_getaltscreen", 1, 1, f_term_getaltscreen},
     {"term_getattr",	2, 2, f_term_getattr},
     {"term_getcursor",	1, 1, f_term_getcursor},
     {"term_getjob",	1, 1, f_term_getjob},