Threaded MPMs for Unix and Win32: Add WorkerStackSize directive

Threaded MPMs for Unix and Win32: Add WorkerStackSize directive
to override default thread stack size for threads which handle
client connections.  Required for some third-party modules on
platforms with small default thread stack size.

This is also useful for trimming back the stack size on
platforms with relatively large default stack size in order to
conserve address space for supporting more threads per child.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@102931 13f79535-47bb-0310-9956-ffa450edef68