[exim] Add plugin for the Exim mail transfer agent

[suraj-cmd]

sos has plugins for postfix, sendmail and dovecot, but nothing for exim, and no
other plugin references it. Exim is the default MTA on Debian and Ubuntu and is
also packaged for Fedora and EPEL, so an sosreport from one of those hosts
currently contains no mail configuration, queue state or logs.

The layout differs enough between distributions to need subclasses: Red Hat
ships a single /etc/exim/exim.conf with logs named main.log, reject.log
and panic.log under /var/log/exim, while Debian uses the split
configuration under /etc/exim4 with mainlog, rejectlog and paniclog
under /var/log/exim4, and the exim4 unit name.

Common to both, the plugin runs exim -bV for the build and configuration
summary and exim -bp for the queue.

The passwd and passwd.client maps hold SMTP authentication credentials in
cleartext, including those used to relay through a smarthost. Both are added to
the forbidden paths in each subclass so they cannot be collected under any
option combination.

I do not have an exim deployment to test against. The Debian log filenames and
the exim4 unit name are taken from the packaging rather than a live system,
so confirmation would be welcome.

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[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo dnf install -y 'dnf*-command(copr)'
• dnf copr enable packit/sosreport-sos-4422
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.

[arif-ali]

again, please test the plugin before hand, we want to ensure that no secrets are coming out of config files or anything like that

[bmr-cymru]

Unfortunately I can't meaningfully test this as I haven't touched exim since it was dropped as the default MTA for RHEL (around RHEL5 era—if you poke around in the legacy sysreport history you can see we had support for it, but no sos plugin was ever created).

[pmoravec]

Can't be there (or in equivalent Debian file) some secret to obfuscate?