Allow malloc debug with dcload-serial

[QuzarDC]

Following #599 , a hole was still left in that the malloc debugging still failed while using dcload-serial. The first hurdle to this was that malloc is used to provide dcload-serial a working buffer for compressed transactions. This happens as early as possible, and so would attempt to use dbgio prior to it being initialized.

The first change here forces dbgio to be disabled until initted, which prevents the (necessarily) silent assert that would be caused by using dbgio before it is initialized.

Unfortunately I found that while this will allow malloc debugging to work through kos' init, it still fails (typically by hard rebooting) somewhere close to the handoff to user code or into there.

Opening this as a draft with the partial progress for feedback and in case anyone else can make more progress with it. There's a chance that the actual issue may be with dcload-serial generally. If there isn't any movement or feedback in a while I'll just move to add it in as the change seems as though it should make fine sense anyways.