Improve a few appendStringInfo calls new to v18

Looks like v18 has grown a few appendStringInfo misusages, e.g. using
appendStringInfo() when no formatting is needed or just using format
"%s" instead of using appendStringInfoString().

I've attached a couple of patches. The 0001 is just my method for
finding these, not for commit. 0002 contains the fixes to commit.

Any objections to doing this soonish? Or in a few weeks?

David