Move spin bit to path context

CI for Windows: integration with AppVeyor

Try to not choke AppVeyor on huge debug log. Redirecting stderr did not
have the intended effect, the command would still fail:

    Command executed with exception:  0

For some reason AppVeyor (VS2017 14.15.26726) fails to link
picoquicdemo due to an issue related to JMC[1]. Apparently JMC requires
an application to be linked with a CRT which somehow does not seem to
happen. JMC can be disabled through "/JMC-" as described at [1].
Ignore the issue until a proper solution is found.

 [1]: https://blogs.msdn.microsoft.com/vcblog/2018/06/29/announcing-jmc-stepping-in-visual-studio/

OpenSSL 1.1.0i on AppVeyor does not include applink.c, so copy it from
OpenSSL 1.0.2p (which has lib\VC\libeay32MD.lib, but not libcrypto.lib).

The weird "2>&1" redirection for git clone tries to prevent PS from
printing a RemoteException (NativeCommandError) from using stderr.

Migration test with ats

Add an option for key logging (draft -14)

The log_labels logic is based on setup_traffic_protection in picotls.c.
Enables QUIC decryption with Wireshark v2.9.0rc0-1850-g2fd42045f5.

The file handle will be leaked on exit, since this is a debug feature I
am not too worried about that.

Implement connection migration

demo: do not crash if server context could not be created

Add logging of retry and negotiation packets in server log.

Remove not working IPv4 DONT FRAG attempt on Linux