os: usingnamespace fixes for std.x.os.Socket and std.os.TCP

Extract existing constants to do with TCP socket options into a 'TCP'
namespace.

Export 'MSG' and 'TCP' from std.os.{linux, windows} into std.c.

Fix compile errors to do with std.x.os.Socket methods related to setting
TCP socket options.

Handle errors in the case that an interface could not be resolved in an
IPv6 address on Windows. Tested using Wine with the loopback interface
disabled.

Have all instantiations of std.x.os.Socket on Windows instantiate an
overlapped socket descriptor. Fixes the '1ms read timeout' test in
std.x.net.tcp.Client. The test would previously deadlock, as read
timeouts only apply to overlapped sockets.

Windows documentation by default recommends that most instantiations of
sockets on Windows be overlapped sockets (s.t. they may operate in both
blocking or nonblocking mode when operated with WSA* syscalls). Refer to
the documentation for WSASocketA for more info.

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