MAC/do: Convert internal TAILQs to STAILQs

[olce]

Summary

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This series has also been squeezed into D47633 to provide an overall view.

Commit message:
We only browse these forward and never need to remove arbitrary elements
from them.

No functional change intended.

[emaste]

It's worth expanding the commit message - even if there's no functional change intended we should say why we're bothering with the change. Presumably it is because we only ever iterate over these in the forward direction so we can avoid the additional overhead.

But, LGTM.

[olce]

In D47624#1086651, @emaste wrote:

It's worth expanding the commit message - even if there's no functional change intended we should say why we're bothering with the change. Presumably it is because we only ever iterate over these in the forward direction so we can avoid the additional overhead.

But, LGTM.

Done.