Add custom thread creation

[huitema]

This PR adds two important features to the network thread support:

1. Support of thread creation in application frameworks that require framework specific APIs instead of using the platform APIs in Windows or the pthread APIs in Unix systems.

2. Support of thread naming, which is difficult to implement in some OSes for which the naming API must be used from inside the named thread (Apple, mostly).

See the definition of picoquic_start_custom_network_thread in the new picoquic_packet_loop.h.

[huitema]

The appveyor tests are failing with a diagnostic "Failed to set up the execution context to run the test". On line search points to something related to dll dependencies. The Visual Studio tests are passing locally, so we get unblocked by just disabling test execution in Appveyor, while still checking that compile succeeds.