list overrides are not allowed

The edit.c code calls fr_pair_list_afrom_str(), which does NOT
handle list over-rides.

Plus, we are assigning to a list as

	&list += "pairs = value"

so having a list override does not make sense.

src/tests/modules/exec/backticks_list.unlang

@@ -10,11 +10,11 @@ if (&Tmp-String-0 != 'hello') {
 }
 
 #
-#  Test default list override
+#  @todo - list over-rides are not allowed!
 #
-&control += `/bin/sh -c "echo reply.Reply-Message := \'hello\'"`
+&control += `/bin/sh -c "echo Reply-Message := \'hello\'"`
 
-if (&reply.Reply-Message != 'hello') {
+if (&control.Reply-Message != 'hello') {
 	test_fail
 } else {
 	test_pass