network: add support for MACVLAN source mode

Add support for creating a MACVLAN interface in "source" mode by
specifying Mode=source in the [MACVLAN] section of a .netdev file.

A list of allowed MAC addresses for the corresponding MACVLAN can also
be specified with the SourceMACAddress= option of the [MACVLAN] section.

An example .netdev file:

    [NetDev]
    Name=macvlan0
    Kind=macvlan
    MACAddress=02:DE:AD:BE:EF:00

    [MACVLAN]
    Mode=source
    SourceMACAddress=02:AB:AB:AB:AB:01 02:CD:CD:CD:CD:01
    SourceMACAddress=02:EF:EF:EF:EF:01

The same keys can also be specified in [MACVTAP] for MACVTAP kinds of
interfaces, with the same semantics.