slirp: Adding IPv6 support to Qemu -net user mode

Hello,

This is another respin of IPv6 in Qemu -net user mode.

These patches add ICMPv6, NDP, and make UDP and TCP compatible with IPv6. We have made some refactoring to make current code compatible with IPv6.

Patches 1 to 8 are refactoring of existing code and do not change the behavior, 9 adds a helper, and 10 to 18 add ipv6 support. Patches 3 and 16 are only reindents of the resulting code, without code change.

Difference with version 3 is:
- reindentation has been moved into separate patches
- inet6 support in sockaddr_equal has been moved to the ndp support patch.
- update first qemu version for the options

Here is a summary of the patches:

Refactoring patches:

[PATCH 01/18] slirp: goto bad in udp_input if sosendto fails [PATCH 02/18] slirp: Generalizing and neutralizing code before adding [PATCH 03/18] slirp: Reindent after refactoring
[PATCH 04/18] slirp: Make Socket structure IPv6 compatible [PATCH 05/18] slirp: Factorizing address translation [PATCH 06/18] slirp: Factorizing and cleaning solookup() [PATCH 07/18] slirp: Make udp_attach IPv6 compatible [PATCH 08/18] slirp: Adding family argument to tcp_fconnect()

Adding helper:
[PATCH 09/18] qemu/timer.h : Adding function to second scale

Adding v6 support:
[PATCH 10/18] slirp: Adding IPv6, ICMPv6 Echo and NDP [PATCH 11/18] slirp: Adding ICMPv6 error sending
[PATCH 12/18] slirp: Adding IPv6 UDP support
[PATCH 13/18] slirp: Factorizing tcpiphdr structure with an union [PATCH 14/18] slirp: Generalizing and neutralizing various TCP [PATCH 15/18] slirp: Reindent after refactoring
[PATCH 16/18] slirp: Handle IPv6 in TCP functions
[PATCH 17/18] slirp: Adding IPv6 address for DNS relay [PATCH 18/18] qapi-schema, qemu-options & slirp: Adding Qemu options