networkd: Add support for bond option.

networkd: Add support for bond option.

This patch adds configurational support for bond option.

Test conf:

bond.netdev

 ---
[NetDev]
Name=bond1
Kind=bond

[Bond]
ArpAllTargets=all
PrimaryReselect=better
ArpIntervalSec=10s
ArpIpTargets= 192.168.8.102 192.168.8.101 192.168.8.102
 ---

$cat /proc/net/bonding/bond1
Ethernet Channel Bonding Driver: v3.7.1 (April 27, 2011)

Bonding Mode: load balancing (round-robin)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0
ARP Polling Interval (ms): 10000
ARP IP target/s (n.n.n.n form): 192.168.8.100, 192.168.8.101, 192.168.8.102