Draft native systemd support

[Andreas Metzler]

Hello,

as required for trixie I have drafted native systemd support for exim
on https://salsa.debian.org/exim-team/exim4/-/commits/2023-native-systemd

Open issue: I suspect that given usrmerge
/lib/systemd/system/exim4.service should actually be installed into
its native location, i.e. /usr/lib/systemd/system/exim4.service. (We are
adding, not moving from /lob to /usr/lib)

cu Andreas
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