networkd: support proxy_arp_pvlan sysctl

The proxy ARP private VLAN sysctl is useful for VLAN aggregation, see
https://sysctl-explorer.net/net/ipv4/proxy_arp_pvlan/ for details.

test/test-network/conf/25-sysctl.network

@@ -7,6 +7,7 @@ IPForward=yes
 IPv6DuplicateAddressDetection=3
 IPv6HopLimit=5
 IPv4ProxyARP=yes
+IPv4ProxyARPPrivateVLAN=yes
 IPv6ProxyNDP=yes
 IPv6AcceptRA=no
 IPv4AcceptLocal=yes

test/test-network/systemd-networkd-tests.py

@@ -3592,6 +3592,7 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         self.check_ipv6_sysctl_attr('dummy98', 'proxy_ndp', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'forwarding', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'proxy_arp', '1')
+        self.check_ipv4_sysctl_attr('dummy98', 'proxy_arp_pvlan', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'accept_local', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'rp_filter', '0')