PauseAndLock was only called with do_lock=true, and the function only
used unpauseOnUnlock when do_lock was false.

Replace calls of FifoManager::PauseAndLock(do_lock=false) with new
function RestoreState for clarity.

Given how many member functions make use of the system instance,
it's likely just better to pass the system instance in on construction.

Makes the interface a little less noisy to use.

This way you can't mix up regular config callback IDs and CPU thread
config callback IDs. (It would be rather bad if you did!)

When faced with this error, users often don't try disabling dual core,
even though the error message suggests it. Perhaps the message is just
too long and lists too many things?

To try to improve the situation, I'm rewording the message and making it
say different things depending on what settings you are using.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

PushFifoAuxBuffer only memcpys data using ptr as the source pointer, so
it can be a pointer to const data because of that.