shared/bus-unit-util: add bus_dump_transient_settings() helper

bus_append_unit_property() and associated functions accept a long list of
properties. But the specific names are only available through code. But it is
useful to be able to know the specific list of properties that is supported, in
particular for shell completions. Thus, add a way to list the properties that
are supported by the code.

In the future we could also turn this into a test for the documentation. For
various reasons, the list of properties listed in the docs is a partially
overlapping set. E.g. for service type, the pull request
https://github.com/systemd/systemd/pull/37661 creates a list with 212 entries,
and this code generates 7 entries less and 184 more. I didn't check all the
differences, but in the few cases I did, the list generated here was actually
correctly supported by 'systemd-run -p'.

A smoke test is added.

src/basic/cgroup-util.c

@@ -2063,6 +2063,10 @@ static const char* const cgroup_io_limit_type_table[_CGROUP_IO_LIMIT_TYPE_MAX] =
 
 DEFINE_STRING_TABLE_LOOKUP(cgroup_io_limit_type, CGroupIOLimitType);
 
+void cgroup_io_limits_list(void) {
+        DUMP_STRING_TABLE(cgroup_io_limit_type, CGroupIOLimitType, _CGROUP_IO_LIMIT_TYPE_MAX);
+}
+
 static const char *const cgroup_controller_table[_CGROUP_CONTROLLER_MAX] = {
         [CGROUP_CONTROLLER_CPU]                             = "cpu",
         [CGROUP_CONTROLLER_CPUACCT]                         = "cpuacct",