patch 8.1.0881: can execute shell commands in rvim through interfaces

Problem:    Can execute shell commands in rvim through interfaces.
Solution:   Disable using interfaces in restricted mode. Allow for writing
            file with writefile(), histadd() and a few others.
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 5857a22..2ca5f2b 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -213,6 +213,7 @@
 	test_regexp_utf8 \
 	test_registers \
 	test_reltime \
+	test_restricted \
 	test_retab \
 	test_ruby \
 	test_scriptnames \
@@ -375,6 +376,7 @@
 	test_quotestar.res \
 	test_regex_char_classes.res \
 	test_registers.res \
+	test_restricted.res \
 	test_retab.res \
 	test_ruby.res \
 	test_scriptnames.res \