iproute2: macvlan: add "source" mode

iproute2: macvlan: add "source" mode

Adjusting iproute2 utility to support new macvlan link type mode called
"source".

Example of commands that can be applied:
  ip link add link eth0 name macvlan0 type macvlan mode source
  ip link set link dev macvlan0 type macvlan macaddr add 00:11:11:11:11:11
  ip link set link dev macvlan0 type macvlan macaddr del 00:11:11:11:11:11
  ip link set link dev macvlan0 type macvlan macaddr flush
  ip -details link show dev macvlan0

Based on previous work of Stefan Gula <steweg@gmail.com>

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
Cc: steweg@gmail.com
v5:
 - rebase and fix checkpatch

v4:
 - add MACADDR_SET support
 - skip FLAG_UNICAST / FLAG_UNICAST_ALL as this is not upstream
 - fix man page