run: fix generated unit name clash after soft-reboot

When sd-run connects to D-Bus rather than the private socket, it will
generate the transient unit name using the bus ID assigned by the D-Bus
broker/daemon. The issue is that this ID is only unique per D-Bus run,
if the broker/daemon restarts it starts again from 1, and it's a simple
incremental counter for each client.
So if a transient unit run-u6.service starts and fails, and it is not
collected (default on failure), and the system soft-reboots, any new
transient unit might conflict as the counter will restart:

Failed to start transient service unit: Unit run-u6.service was already loaded or has a fragment file.

Get the soft-reboot counter, and if it's greater than zero, append it
to the autogenerated unit name to avoid clashes.

test/units/testsuite-82.sh

@@ -146,6 +146,11 @@ elif [ -f /run/testsuite82.touch ]; then
     # Restart the unit that is not supposed to survive
     systemd-run --collect --service-type=exec --unit=testsuite-82-nosurvive.service sleep infinity
 
+    # Now ensure there are no naming clashes and a bunch of transient units all succeed
+    for _ in $(seq 1 25); do
+        systemd-run --wait true
+    done
+
     # Now issue the soft reboot. We should be right back soon. Given /run/nextroot exists, we should
     # automatically do a softreboot instead of normal reboot.
     touch /run/testsuite82.touch2
@@ -227,6 +232,14 @@ EOF
         systemd-inhibit --what=shutdown --who=test --why=test --mode=delay \
             sleep infinity
 
+    # Enqueue a bunch of failing units to try and trigger the transient name clash that happens due to D-Bus
+    # being restarted and the "unique" bus IDs not being unique across restarts
+    for _ in $(seq 1 25); do
+        # Use --wait to ensure we connect to the system bus instead of the private bus (otherwise a UUID is
+        # used instead of the bus ID)
+        systemd-run --wait false || true
+    done
+
     # Now issue the soft reboot. We should be right back soon.
     touch /run/testsuite82.touch
     systemctl --no-block --check-inhibitors=yes soft-reboot