core: Apply ap_max_mem_free to created threads' pool allocator.

core: Apply ap_max_mem_free to created threads' pool allocator.

Since APR does not set the threshold above which the allocator of the thread's
starts returning its memory to the system, so set ap_max_mem_free from
ap_thread_create(), ap_thread_main_create() and ap_thread_current_create().

* include/httpd.h:
  Provide our own ap_thread_create() in any case (but !APR_HAS_THREADS).
  Simplify #ifdef-ery.

* server/util.c(thread_start, ap_thread_main_create, ap_thread_current_create):
  Set ap_max_mem_free to the thread's pool allocator.
  Simplify #ifdef-ery.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1902728 13f79535-47bb-0310-9956-ffa450edef68