networkd: introduce veth device support

This patch adds veth device support to networkd.

Example conf:

File: veth.netdev

[NetDev]
Name=veth-test
Kind=veth

[Peer]
Name=veth-peer
networkd-netdev: fix white space
networkd: introduce gre tunnel

This patch enables gre tunnel support.

example conf:

file : gre.netdev
[NetDev]
Name=gre-tun
Kind=gre
MTUBytes=1480

[Tunnel]
Local=X.X.X.X
Remote=X.X.X.X

file: gre.network
[Match]
Name=em1

[Network]
Tunnel=gre-tun

TODO:

Add more attributes for gre tunnel
IFLA_GRE_IFLAGS
IFLA_GRE_IFLAGS
IFLA_GRE_IKEY
IFLA_GRE_OKEY
networkd: sit-tunnel add support for pmtudisc

This patch adds path of mtu discovery for sit tunnel.
To enable/disable DiscoverPathMTU is introduced.

Example configuration

file: sit.netdev
[NetDev]
Name=sit-tun
Kind=sit
MTUBytes=1480

[Tunnel]
DiscoverPathMTU=1
Local=X.X.X.X
Remote=X.X.X.X

By default pmtudisc is turned on , if DiscoverPathMTU
is missing from the config. To turn it off
DiscoverPathMTU=0 needs to be set.
networkd: introduce vti tunnel

This patch enables vti tunnel support.

example conf:

file : vti.netdev
[NetDev]
Name=vti-tun
Kind=vti
MTUBytes=1480

[Tunnel]
Local=X.X.X.X
Remote=X.X.X.X

file: vti.network
[Match]
Name=em1

[Network]
Tunnel=vti-tun

TODO:

Add more attributes for vti tunnel
IFLA_VTI_IKEY
IFLA_VTI_OKEY