Add support for external pthreads lib on windows and some docs about how to do.
It can build with LWS_WITH_THREADPOOL and LWS_WITH_MINIMAL_EXAMPLES including the
pthreads-dependent ones without warnings or errors on windows platform as well with this.
pthreads_t can be anything, including a struct - not a pointer-to-a-struct
but the struct itself. These can't be cast to a void * for printing as they can
on linux, where the base type is a pointer.
Let's fix all the usage of those to determine their own thread index in terms
of the meaning to the program rather than as a tid.
This extends threadpool slightly so it can bind enqueued tasks to a
secure streams handle as well as a straight wsi.
Either the .wsi must be set as before, or the .ss handle if you are
using secure streams, when enqueuing a task on the taskpool.
A couple of other helpers get ss-aware wrappers if LWS_WITH_SECURE_STREAMS
Although threadpool was originally designed for server (gitohashi)
actually it's also fine working with client wsi / Secure Streams,
if you have a situation a client connection is associated with heavy
processing.
