core: introduce SuccessAction= as unit file property

SuccessAction= is similar to FailureAction= but declares what to do on
success of a unit, rather than on failure. This is useful for running
commands in qemu/nspawn images, that shall power down on completion. We
frequently see "ExecStopPost=/usr/bin/systemctl poweroff" or so in unit
files like this. Offer a simple, more declarative alternative for this.

While we are at it, hook up failure action with unit_dump() and
transient units too.

src/shared/bus-unit-util.c

@@ -408,7 +408,7 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
                               "RootDirectory", "SyslogIdentifier", "ProtectSystem",
                               "ProtectHome", "SELinuxContext", "Restart", "RootImage",
                               "NotifyAccess", "RuntimeDirectoryPreserve", "Personality",
-                              "KeyringMode", "CollectMode"))
+                              "KeyringMode", "CollectMode", "FailureAction", "SuccessAction"))
                 r = sd_bus_message_append(m, "v", "s", eq);
 
         else if (streq(field, "StandardInputData")) {