logs: Use journalctl -o with-unit for default logs

Before systemd was created, syslog was the dominant Unix logging
system.

Later, systemd introduced the journal whose output intentionally
exactly matched that of syslog for compatibility.

In systemd, the "unit" is the technical heart of things; yet
the syslog output only includes the "syslog identifier" which
might or might not look like the unit.

For many units that just run shell script, the syslog
identifier is just "bash" which is not useful.

The `-o with-unit` is in RHEL8 and above; it's not in RHEL7.  I don't
know if it is a goal of this project to ship the latest releases
on very old operating systems.  If it is, we can try to do dynamic
detection.

sos/report/plugins/logs.py

@@ -130,6 +130,10 @@ def setup(self):
             self.add_cmd_output("journalctl -o export")
         else:
             days = self.get_option("log_days", 3)
-            self.add_journal(since="-%ddays" % days)
+            # The default journal output is syslog-compatible, but it's much
+            # more useful to have the systemd unit.  Related prior art:
+            # https://github.com/openshift/installer/pull/7371/commits/948df48aada21e47e9bb0d0a53366871aa21b40a
+            # https://github.com/coreos/coreos-assembler/commit/c931fe0e9aa6b7c20c8279fdce3b6ef448eac8b8
+            self.add_journal(since="-%ddays" % days, output="with-unit")
 
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