netifd: vxlan: add most missing boolean options

netifd: vxlan: add most missing boolean options

adds the folloing missing options:
  - learning
  - rsc
  - proxy
  - l2miss
  - l3miss
  - gbp

See ip-link(3) for their meaning.

still missing:
  - external
  - gpe

I'm not sure how to handle them at the moment. It's unclear to me what
IFLA_VXLAN_* value corresponds to the 'external' option and according to
the manpage, gpe depends on it.

Signed-off-by: Johannes Kimmel <fff@bareminimum.eu>