[libmicrohttpd] pthread stack size

[Eivind Sarto]

When threads are created by libmicrohttpd, they are created with default thread 
attributes.
The default thread stack is quite large.  It different on each architecture.  
On x86_64, it is 10MB per thread.
That starts adding up if you create a thread per connection and you have a lot 
of connections.

I am evaluating if I can use libmicrohttpd for a project that will require up 
to 10000 simultaneous connections.
That would require 100GB of stacks.

I am adding an option MHD_OPTION_THREAD_STACK_SIZE to the library to allow 
threads to be created with less stack.
That will allow me to run some simulations.

That option could be useful for others who will use lots of threads.
Maybe it could be added to the sources?

-eivind