Use '$initial_call' instead of initial_call

BugzID: 18742
diff --git a/src/rexi_server.erl b/src/rexi_server.erl
index 49661cc..7d928ad 100644
--- a/src/rexi_server.erl
+++ b/src/rexi_server.erl
@@ -146,7 +146,7 @@
     string() | undefined) -> any().
 init_p(From, {M,F,A}, Nonce) ->
     put(rexi_from, From),
-    put(initial_call, {M,F,length(A)}),
+    put('$initial_call', {M,F,length(A)}),
     put(nonce, Nonce),
     try apply(M, F, A) catch exit:normal -> ok; Class:Reason ->
         Stack = clean_stack(),