meson: rework processing of unit files

Ideally, we would chain the m4 processing, .in substitutions, and file
installation so that the commands don't have to be repeated. Unfortunately
this does not seem currently possible, because custom_target() output cannot
be fed into install_data(), so it's necessary to use the 'install',
'install_dir' arguments to control installation. Nevertheless, rework the
rules to repeat less stuff and unify handling of conditions between the
different file types.

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