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/ex_docmd.c b/src/ex_docmd.c
index b90ea7b..ccca2f9 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2007,11 +2007,16 @@
 #ifdef HAVE_SANDBOX
 	if (sandbox != 0 && !(ea.argt & SBOXOK))
 	{
-	    /* Command not allowed in sandbox. */
+	    // Command not allowed in sandbox.
 	    errormsg = _(e_sandbox);
 	    goto doend;
 	}
 #endif
+	if (restricted != 0 && (ea.argt & RESTRICT))
+	{
+	    errormsg = _("E981: Command not allowed in rvim");
+	    goto doend;
+	}
 	if (!curbuf->b_p_ma && (ea.argt & MODIFY))
 	{
 	    /* Command not allowed in non-'modifiable' buffer */