Update davici to support Windows

[KevinLussier]

These changes add in Windows support to davici. The main change involves using a TCP socket instead of a UNIX socket. Platform dependent types and macros were added:

• sockerr because Windows does not use errno on socket errors but instead uses WSAGetLastError().
• is_recoverable because Windows WSAE* instead of errno for socket errors.
• closesocket because Windows does not support close() for sockets.
• davici_fd because SOCKETs on Windows are different sizes depending on Win32 vs Win64.

Additionally:

• some explicit casts were added to remove compiler warnings on Windows.
• some variable types were changed for the same reason (e.g. signed/unsigned warnings)

[martinwilli]

Generally, I'd prefer to see some platform abstraction header, where we can put away all the differences between Windows and Unix systems.

[martinwilli]

Also, it would be nice to have some integration testing for the Windows build, as otherwise we likely break it with future changes. For strongSwan we use MinGW on Travis Linux hosts, that would certainly be an option. AppVeyor would be a good alternative, which Tobias added to strongSwan, too.

[martinwilli]

I'm not so happy with all those added casts, as they make the code rather unreadable. They mostly just silence some warnings; but maybe we can disable just those compiler warnings?