document ability to disable MACAddressPolicy

While it is currently possible to either not set MACAddressPolicy or set
it to a value different from "persistent" or "random", it is not obvious
that a user can do so. Add a policy, "none", which simply retains kernel
MAC addresses (same as not filling in the policy at all) and document it
so that users are aware of this setting.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>

man/systemd.link.xml

@@ -233,6 +233,12 @@
                 <literal>locally administered</literal> bits set.</para>
               </listitem>
             </varlistentry>
+            <varlistentry>
+              <term><literal>none</literal></term>
+              <listitem>
+                <para>Keeps the MAC address assigned by the kernel.</para>
+              </listitem>
+            </varlistentry>
           </variablelist>
         </listitem>
       </varlistentry>

src/udev/net/link-config.c

@@ -460,6 +460,7 @@ int link_config_apply(link_config_ctx *ctx, link_config *config,
                                 mac = &generated_mac;
                         }
                         break;
+                case MACPOLICY_NONE:
                 default:
                         mac = config->mac;
         }
@@ -492,7 +493,8 @@ int link_get_driver(link_config_ctx *ctx, struct udev_device *device, char **ret
 
 static const char* const mac_policy_table[_MACPOLICY_MAX] = {
         [MACPOLICY_PERSISTENT] = "persistent",
-        [MACPOLICY_RANDOM] = "random"
+        [MACPOLICY_RANDOM] = "random",
+        [MACPOLICY_NONE] = "none"
 };
 
 DEFINE_STRING_TABLE_LOOKUP(mac_policy, MACPolicy);

src/udev/net/link-config.h

@@ -32,6 +32,7 @@ typedef struct link_config link_config;
 typedef enum MACPolicy {
         MACPOLICY_PERSISTENT,
         MACPOLICY_RANDOM,
+        MACPOLICY_NONE,
         _MACPOLICY_MAX,
         _MACPOLICY_INVALID = -1
 } MACPolicy;