Refactor DebuggerCommand.

Rename onClient to onClientImpl and move to protected space. Rename onClientD to onClient. Add comments.

hphp/runtime/eval/debugger/cmd/cmd_shell.cpp

@@ -49,8 +49,8 @@ bool CmdShell::help(DebuggerClient *client) {
   return true;
 }
 
-bool CmdShell::onClient(DebuggerClient *client) {
-  if (DebuggerCommand::onClient(client)) return true;
+bool CmdShell::onClientImpl(DebuggerClient *client) {
+  if (DebuggerCommand::onClientImpl(client)) return true;
   if (client->argCount() == 0) {
     return help(client);
   }