patch 8.1.0615: get_tv function names are not consistent

Problem:    Get_tv function names are not consistent.
Solution:   Rename to tv_get.
diff --git a/src/list.c b/src/list.c
index 679276c..2a82023 100644
--- a/src/list.c
+++ b/src/list.c
@@ -403,7 +403,7 @@
 	    *errorp = TRUE;
 	return -1L;
     }
-    return (long)get_tv_number_chk(&li->li_tv, errorp);
+    return (long)tv_get_number_chk(&li->li_tv, errorp);
 }
 
 /*
@@ -420,7 +420,7 @@
 	EMSGN(_(e_listidx), idx);
 	return NULL;
     }
-    return get_tv_string(&li->li_tv);
+    return tv_get_string(&li->li_tv);
 }
 
 /*
@@ -949,7 +949,7 @@
 
     for (li = list->lv_first; li != NULL; li = li->li_next)
     {
-	for (s = get_tv_string(&li->li_tv); *s != NUL; ++s)
+	for (s = tv_get_string(&li->li_tv); *s != NUL; ++s)
 	{
 	    if (*s == '\n')
 		c = putc(NUL, fd);