networkd: add support for bond transmit hash policy

networkd: add support for bond transmit hash policy

This patch adds support  the transmit hash policy to use
for slave selection in balance-xor, 802.3ad, and tlb modes

layer2, layer3+4, layer2+3, encap3+4, encap3+4

Added:
1. BondXmitHashPolicy
2. conf param TransmitHashPolicy

Test conf:
[NetDev]
Name=bond1
Kind=bond

[Bond]
Mode=802.3ad
TransmitHashPolicy=layer2+3

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

Bonding Mode: IEEE 802.3ad Dynamic link aggregation
Transmit Hash Policy: layer2+3 (2)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0

802.3ad info
LACP rate: slow
Min links: 0
Aggregator selection policy (ad_select): stable
bond bond1 has no active aggregator

[tomegun: dropped bond_ prefix from new Bond variable, drop repeated man-page section]