Add TUNNEL_GRE_FLAGS support for ALL flags supported by ip-link(8) including ignore-df

[archiecobbs]

TUNNEL_GRE_FLAGS supports [i|o]seq and [i|o]csum, but not any other flags that gre interfaces can have, such as ignore-df (see ip-link(8)).

I have an interface gre1 that needs to have the ignore-df flag set, but there's no way to configure this with wicked.

I tried added it to TUNNEL_GRE_FLAGS but got this:

$ wicked ifreload gre1
wicked: ifcfg-gre1: Unsupported TUNNEL_GRE_FLAGS="ignore-df"
wicked: skipping gre1 set-up: no configuration available

TUNNEL_GRE_FLAGS should support all flags supported by ip-link(8), including ignore-df.