netifd: add segment routing support

netifd: add segment routing support

seg6_enabled - Bool
  Accept or drop SR-enabled IPv6 packets on this interface.

More Information:
https://www.kernel.org/doc/html/latest/networking/seg6-sysctl.html

Now you can set as interface option
  option ip6segmentrouting '1'

It is not enough to turn on "seg6_enabled" on the interface. Further,
we have to enable "/all/seg6_enabled". This means that a working config
is "interface + all".

Signed-off-by: Nick Hainke <vincent@systemli.org>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [style fixes]