GRE over IPv6

GRE over IPv6

Refactors the GRE node to work with both IPv4 and IPv6 transports.

Note that this changes the binary configuration API to support both
address families; each address uses the same memory for either
address type and a flag to indicate which is in use.

The CLI and VAT syntax remains unchanged; the code detects whether
an IPv4 or an IPv6 address was given.

Configuration examples:

IPv4 CLI: create gre tunnel src 192.168.1.1 dst 192.168.1.2
IPv6 CLI: create gre tunnel src 2620:124:9000::1 dst 2620:124:9000::2

IPv4 VAT: gre_add_del_tunnel src 192.168.1.1 dst 192.168.1.2
IPv6 VAT: gre_add_del_tunnel src 2620:124:9000::1 dst 2620:124:9000::2

Change-Id: Ica8ee775dc101047fb8cd41617ddc8fafc2741b0
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>