Various changes made as part of trying to run include-what-you-use on the systemd...

Various changes made as part of trying to run include-what-you-use on the systemd repository (#37112)

I have been trying to run
https://github.com/include-what-you-use/include-what-you-use on the
systemd repository to hopefully get a handle on the rampant numbers of
includes we have in every file with no idea if any of the symbols coming
from that file are used or not.

While I haven't got it fully working yet, these changes still make sense
IMO and can be merged already.

Except the last commit, all other changes are about removing circular
dependencies between headers which trips up include-what-you-use.
Regardless of the tool, circular dependencies between headers are a code
smell and I think we should get rid of them regardless of whether we end
up using the tool or not.