internal/uevent: enumerate objects via sysfs

This is not a great way to implement cold boot, but I already have the implementation lying around.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/uevent/message_test.go

@@ -110,6 +110,12 @@ SEQNUM=780`},
 		}, "move", uevent.MissingHeaderError(
 			"move",
 		), syscall.EINVAL, "unsupported kobject_action 2989 event:"},
+
+		{"synthetic", uevent.Message{
+			Action: uevent.Synthetic,
+		}, "synthetic@\x00", uevent.UnsupportedActionError(
+			"synthetic",
+		), nil, "synthetic event:"},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {