Use my_vsnprintf extension %sQ where appropriate

The difference between literal-`%s` and %sQ is that the latter also “escapes” `’s into ``’s.

Not all my_vsnprintf uses make use of this failsafe.
Messages that prompt SQL content (e.g., database identifiers) should use %sQ’s so the quote is copy-pastable as-is.

As MDEV-21978 updates, its Part 2 PR will make a list of my_vsnprintf-delegating functions visible from its changed files.
I’ve marked MDEV-21978 as blocking this anyway as it adds %sQ (that deprecates the old %`s), available in preview soon™.

Markdown formats back`tick as ``back`tick``, but I don’t think we care about those.