core: add ManagedOOM*= properties to configure systemd-oomd on the unit

This adds the hook ups so it can be read with the usual systemd
utilities. Used in later commits by sytemd-oomd.

src/basic/cgroup-util.c

@@ -2161,3 +2161,10 @@ CGroupMask get_cpu_accounting_mask(void) {
 bool cpu_accounting_is_cheap(void) {
         return get_cpu_accounting_mask() == 0;
 }
+
+static const char* const managed_oom_mode_table[_MANAGED_OOM_MODE_MAX] = {
+        [MANAGED_OOM_AUTO] = "auto",
+        [MANAGED_OOM_KILL] = "kill",
+};
+
+DEFINE_STRING_TABLE_LOOKUP(managed_oom_mode, ManagedOOMMode);