networkd veth: Make kind assert

It's more appropriate to make it's assert than -ENOTSUP
networkd: add support for mode

This patch adds supports networkd to configure bond mode
during creation via persistent conf. Mode can be configured
with conf param 'Mode'. A new section Bond is added to the
conf to support bond mode.

These modes can be configured now.

balance-rr
active-backup
balance-xor
broadcast
802.3ad
balance-tlb
balance-alb

Example conf file: test-bond.conf
[NetDev]
Name=bond1
Kind=bond

[Bond]
Mode=balance-xor

Test case:
1. start networkd service:

12: bond1: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc
noqueue state UNKNOWN mode DEFAULT group default
link/ether 22:89:6c:47:23:d2 brd ff:ff:ff:ff:ff:ff

2. find bond mode:

cat /proc/net/bonding/bond1
    Ethernet Channel Bonding Driver: v3.7.1 (April 27, 2011)
    Bonding Mode: load balancing (xor)
    Transmit Hash Policy: layer2 (0)
    MII Status: up
    MII Polling Interval (ms): 0
    Up Delay (ms): 0
    Down Delay (ms): 0

Changes:
       1. Added file networkd-bond.c
       2. Bond mode enum BondMode
       3. conf section [Bond]

[tomegun: whitespace]
networkd todo : remove tun/tap

Remove tun/tap from todo
networkd TODO: add more bonding options

Add more bonding option in TODO section