RemoteDevice: added {keep,remove}_on_down functions

Added functions that can set/unset device's flag to
either keep or remove IPv6 addresses on shutting-down
device.

lnst/Controller/Namespace.py

@@ -265,3 +265,9 @@ def _update_device_id(self, value, name):
             return
 
         dev._id = name
+
+    def keep_addrs_on_dev_down(self, device: Device) -> None:
+        self.run(f"echo 1 > /proc/sys/net/ipv6/conf/{device.name}/keep_addr_on_down")
+
+    def remove_addrs_on_dev_down(self, device: Device):
+        self.run(f"echo 0 > /proc/sys/net/ipv6/conf/{device.name}/keep_addr_on_down")

lnst/Devices/RemoteDevice.py

@@ -188,6 +188,13 @@ def __repr__(self):
 
         return "{}({})".format(self._dev_cls.__name__, ", ".join(args))
 
+    def keep_addrs_on_down(self) -> None:
+        self.netns.run(f"echo 1 > /proc/sys/net/ipv6/conf/{self.name}/keep_addr_on_down")
+
+    def remove_addrs_on_down(self):
+        self.netns.run(f"echo 0 > /proc/sys/net/ipv6/conf/{self.name}/keep_addr_on_down")
+
+
 class PairedRemoteDevice(RemoteDevice):
     """RemoteDevice class for paired Devices (such as veth)"""
     def __init__(self, peer, dev_cls, args=[], kwargs={}):