cgroup: Add support for ProtectControlGroups= private and strict

This commit adds two settings private and strict to
the ProtectControlGroups= property. Private will unshare the cgroup
namespace and mount a read-write private cgroup2 filesystem at /sys/fs/cgroup.
Strict does the same except the mount is read-only. Since the unit is
running in a cgroup namespace, the new root of /sys/fs/cgroup is the unit's
own cgroup.

We also add a new dbus property ProtectControlGroupsEx which accepts strings
instead of boolean. This will allow users to use private/strict via dbus
and systemd-run in addition to service files.

Note private and strict fall back to no and yes respectively if the kernel
doesn't support cgroup2 or system is not using unified hierarchy.

Fixes: #34634

src/shared/bus-unit-util.c

@@ -1047,6 +1047,7 @@ static int bus_append_execute_property(sd_bus_message *m, const char *field, con
                               "ProtectHome",
                               "PrivateTmpEx",
                               "PrivateUsersEx",
+                              "ProtectControlGroupsEx",
                               "SELinuxContext",
                               "RootImage",
                               "RootVerity",